I use custom textures for a construction with SurfaceAppearance but the resolution on roblox when I upload the textures is very low. I do not know if it comes from my textures or if roblox does not accept 2048 x 2048.
The resolution limit is currently capped at 1024x1024 there is no way you could actually upload a higher resolution the only solution is to slice the parts of the model with different textures modify the UV map and make 2 or 4 textures out of it
There’s no automatic way to do this either so you should take this in consideration when making your next meshes