Tweener: make a small bike big, or a big bike small

Pretty much yes to this.  I would add that ideal reach numbers can also shift based on the style/category of bike and the terrain.  For example, I tend to want to be on the shorter side of my ideal reach on a bike where I am doing a lot of seated pedaling (think xc bike or short travel trail bike) or low gradient riding with mixed speeds.  On an all mountain bike, I start to head towards the middle/longer side of my ideal reach range to get a bit of stability.  Same goes for my enduro bikes.  Once I get to ebikes or dh bikes, I start to trend back shorter, based mostly on weight distribution and wheelbase.  

Kind of along with this, assuming you are buying a bike, if you are really stuck on which of two sizes you want to get a bike in, it's probably a good idea to start looking at other bikes as well. Look around at other brands that have bikes that suit your riding, odds are that there is a bike out there that you won't be compromising on fit wise, it just might take some time to find it. Let's not get hung up on specific models and brand loyalties if there is something else out there that will work. Do you really want that Stumpy or do you want a good mid travel trail bike that suits you? 

The trail bike thread next door has me looking at a few bikes again (Optic, Spur), and with my 177cm / something like 5'10"  I once again can't make up my mind on what size I'd want... Ended up sizing down to an M on my Propain Tyee and I like it that way, but some brands M bikes are even smaller while the L is longer than anything I've ever owned (and I find my M/L hardtail already quite long).
Ideal reach would probably be in the 465mm area.

Maybe another few more cased jumps and I'll get short enough to not be between two sizes !

There is, depending on the bike, some ability to adjust reach and stack with a headset (and stem), but that’s not a ton of adjustment. Definitely something like to see more of when there’s a wide gap between sizes. 

If it's a very compact bike, such as a short travel ripper, I'd lean towards the larger size.

If it's a very long bike, such as some gravity sled, I'd lean towards the smaller size.

This is just in general. Of course I'd consider a lot more than this, as I'd like to make full use of all the context I can get for my big decisions.

It's interesting reading these comments. And I'm reminded how different the sizing recommendations from almost every brand are from the 'shorter' crowd is here. Like the idea that at 5'10" you're not a large is ridiculous. That's an inch above average American male height. Obviously large. 

And I would venture to guess that the 'smaller is better' and 'can't find front end grip' crowd are all riding a bit old school. As in not riding centered on the bike as modern geometry and sizing is intending. 

As a former longtime motorcycle rider on sport bikes I'm all about that front brake and loading into the front during corners and at 5'11", +1" ape and 32" inseam my bike at 485 with maximum spacers and a 50mm riser bar rolled way forward I could run longer. And flickability and handling improve with higher stack. Tight turns are about deeper lean, not a shorter wheelbase. I ride tight trails and love the maneuverability of my bike. 

Remember folks, from 480mm to 510mm is barely more than an inch. Get out a tape measure sometime, it's such a tiny difference yet feels so much more composed with good technique. 

My 2 favorite bikes, a 510mm reach dh bike for hauling the mail on bike park tracks that are mostly wide and fast with lots of line choice and a 469mm reach e-bike that's for trying to haul the mail on tight, twisty tech trails where line choice isn't always a thing but quick reactions is definitely a bonus. I'm just over 6'3" with a +3 ape index. Both bikes are mullet, XL size and with vastly different purposes.

Short reach, high stack and long stays and you are golden. 

No but for real, firstly look at wheelbase, which should be somewhere between 1230 an 1260, and then kinda pick on preference, longer if you want more "stabiliity" and well shorter if you want to have fun. 

I tried both extremes at 178cm and both is doable. 

Previous bike 475 reach, shortened to probably around 470 due to overforking and slamming it in to low, which pushed the wheelbase right to 1260 with 63° head angle (with 140 mm travel out back) and 440 something stays. 

Bike at least from geometry stand point was superb as long as you had some support in corners. On flat corners there was alot of mental and usual gymnastics to make it work, but other than that bike ripped. 

Now on 450 reach 450 stays, with 1230ish wheelbase and honestly, despite it being a slightly shorter and steeper bike (65° degree head angle) I do not notice any lack of stability even on super janky super steep, shouldn't be ridden on 135mm bike kind of shit. And flat corners went from my number one weakness to one of the biggest strengths, but othervise I don't think it is much better in tight tech stuff so yeah, take from that you want 

To summarize: 1230 to 1260 wheelbase and try to keep front to rear ratio somewhere between 1.75 to 1.85 and you should be good. In the end we adapt to everything. Personaly I would err on shorter size. but mostlly due to my riding style and type of shit I like to ride. 

XL with 469 reach? In what world?

YT Decoy MX with the Shimano ep801 motor. The bike was not changed for many years (other than motor and battery package) so its sizing was pretty old fashioned. Great for my old trails. 

At 178cm I'm also always between M and L on the sizing charts, here is my two cents:

Going back to 2014, my YT Capra was a Medium, and the reach ( just checked ) was only 422mm; I was then a medium also by their sizing charts.

In the last decade and a bit, bikes didn't only get longer, but the sizing charts usually also send us to a larger size, so if I follow some recommendations, I should be riding an L bike on most manufacturers which will usually have now 475-480 reach. That's a big change!

Since then, I've been riding a bunch of YT's, including Capras, Jeffseys, and Izzos, both M and L, I had a few Commencal Meta trail and AM, a Merida 975 size L, a Stif Squatch size M, a Transition Sentinel Size M, and I also test-riden Trek Fuel EX, Haro Daley and Kona Honzo ESD...

What I found again and again is that for me, a reach which is as close to 460-465 just feels very natural. I can handle large bikes, but they do feel a bit less fun, and a bit more stable naturally.

What in the end helps me make a decision is riding style, and how fast I go on the trails.
I like technical riding, steep stuff, an occasional bike part visit, but also mellow flow trails - and on all of those, I never ride super fast - nothing close to race pace, which is where I think bike designers today take most of their inspiration from.

I could probably gain stability on fast straight lines on a large bike, but I don't care about that really, and I can go faster than I'm comfortable with on any modern medium bike.

The industry made two changes: first one is longer bikes, which is great - I wouldn't want to go back to the 422 reach on my 2014 Capra ( which was still amazing for the day ) , second one it sending us all to a larger size if we're on the limit. If you still want to play with the bike, do some small pops and manuals on the trail and have fun without hauling it full speed - find the reach sizing that feels good to you and stick to around that. 

Does anybody here have experience with the Norco Shore? I'm looking at two options that are each medium and large. I'm 5'10 with a long inseam and not insignificant ape index, so I'm curious if the medium would work. It's also cheaper so that might be the deciding factor anyways.