I also want this. The texture and mask tools have had snap to grid for years now. Why is this the one feature that doesn't have it? I do have an axe to grind with this because the resize tool simply does not accept precision! I even did the math to calculate how large each of my grid squares was, and multiplied that by the length/height of the selection I wanted to make, and it wasn't even close!

Exactly, I was going to post this one myself but found yours! I keep resizing maps e.g. when I'm expanding them to have a few floors in one file or to avoid unnecessary big surroundings and the frustration when the grid doesn't match the map... geez