The problem with most game stock art is that it is often stunningly ignorant of modern rendering inputs. If I'm getting a stock model, I need versions at multiple poly counts, with diffuse, bump, normal, gloss/specular, and maybe a few other maps. Similarly, stock textures have little value without matching bump/normal/gloss maps. And all those maps need to be at 4k sizes or better.

I think it depends some on the game though...

some of us only occasionally use bump and normal maps, and consider 256x256 or similar to be "good enough for most things" (well, nevermind all the 64x64 and 128x128 textures). a partial exception is using 2k x 2k for a grass texture, but mostly because it is spread out over a 16-meter area (leaving it at 128x128 pixels per square meter), and this was mostly to reduce an ugly-looking obvious tiling effect on the terrain.

some of this may be due to matters like "how much space is reasonable", like how willing people might be to download an "indie" game if it requires multiple GB for all the art-assets or similar (vs, say, 100MB, 200MB, or maybe 400MB...). it also helps reduce the issue of people complaining because it doesn't download in their browser without the download crapping out, and them refusing to use a download-manager because it is somehow the developers' responsibility to make the download not crap out if they download it directly in their web-browser, ...

ADD/EDIT: basically, 128 or 256 pixels per-meter seems to work acceptably well, and the exact size of a texture can depend some on the size of the area being covered (say, 2k or 4k pixel for a 16 meter area, or 1k for a 2 or 4 meter area, ...).

(ADD 2 / Note: generally assuming a first-person POV with the player usually being "reasonably close" to the geometry, further away needs less detail, and a player with their face against a wall will still be able to see the individual pixels at 128 or 256 px/m...).

potentially though higher resolution textures could be available for those willing to download them.