XC Bike Sizing - Long is the new fast?

Question to those who have endeavored to try a long reach XC bike - how is it working out for you? I’m 5’10 and usually in-between sizes, and I have historically sized down on my XC bikes (eg size M 2021 specialized epic fit just fine). However I’m absolutely in love with how my L Rocky Mountain Altitude fits/rides with its 470 reach, and I’m tempted to try a size L Specialized Epic with the same length. Obviously the bikes are for VERY different purposes so I’m wondering if I’m making a mistake by sizing up here.

Any downsides I’m not considering?

What’s the STA difference between the two?

The risk is you’ll end up with a more stable XC bike that doesn’t feel as nimble at lower speeds/in the tight stuff.

I’m 5’10ish 5’11ish on a L Epic Evo and have historically ridden M everything (sb130, ripmo, etc). My saddle is a bit slammed forward and i’m using a 50mm stem and it feels good, especially when pointed down. The slacker seat tube angle means the top tube is longer and I felt a bit stretched out while pedaling before shifting the saddle forward. YMMV

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Yeah, long wheelbase or long stem? If it’s super tight a short wheelbase will be faster. If it’s narrow (tree) trails or you’re doing something closer to gravel biking, longer stem / narrower bars will be faster. If you’re going feature-rich trails or bumpy descents, you probably want shorter stem, wider bars, and longer wheelbase (slacker).

FWIW, as long as your seatpost is within range and standover height is ok, I don’t think sizing is as critical on a MTB as it is on a road bike. The difference in sizes on a MTB is basically the reach and seat tube height; the head tube length is a bit different but not all that much. The fit is so different compared to a road bike, with the MTB so much more compact. If you’re using an XC bike like a gravel bike rather than a (feature rich) trail bike, the fit begins to matter more as your fit will be pretty close to your road bike.

On my personal bike, I bought a new Chisel, with a 50mm stem (which is basically directly on the steerer). I ride a 58cm Trek H2 road bike with a 120mm stem. Spec said L but I found the L was WAY too short for my liking. I got an XL and put a 100mm stem on it slammed all the way. The other reason for the XL was that I could fit two water bottles in the frame.

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I’m 5’10" on a L Epic EVO. Seat in the middle position, 60mm stem, and 760mm bars. Feels OK to me. You could check Specialized’s fit calculator. It will have you take some measurements and recommend a size. For me that was a Large.