You can load .jpg, .png & .tiff (i think that's all supported, you can load .gif as url on image panels...but that doesn't count as image/texture slot)
.jbg is universal, tbh great choice to save on size (without much quality loss, at ~half the size of .png & .tiff)
for alpha textures or when you need transparency you gonna use .png (obviously), but for everything else stick to .jbg.
It's true, you can apply any size (ratio), but if image is not 1:1 it's gonna be of (especially on flat stuff: floors, walls, cubes...etc)
You gonna need to use tile/offset sliders to reposition & resize the image (unless is some painting on the wall, maybe 16:9, and you wanna use unusual ratio).
I guess overall size depends on texture, how much details?, do you use normals(bumpMap)?, do you care about file size?, is it in foreground or background?
1k - should be the minimum (lowest quality, smallest file size)
2k - is good enough (average)
4k - if you wanna details (when you don't care about file size)
8k - go for it only if you give zero f* (..idk what else to say, just don't go ALL.png)
Loading 8k floor/wall textures is just waste of everything...especially if in background.