Texel density is the term you want. It refers to the ratio/size of the texture's pixels (texels) to the physical geometry. Plenty of threads about it you can search for, and on the wiki: http://wiki.polycount.com/wiki/Texel
Cool more info ! tagging this on my thread for future reference. @Easton you should check it out as it may help you understanding TD more. http://polycount.com/discussion/192294/texel-density-and-units-in-depth#latest
You still want to work in powers of 2 textures. 16, 32, 64, 128, 256, 512 ect. That has to do with how textures are stored and how hardware functions. You don't want to just pick a size 512px and use it on everything. Things that are huge like a building will look horrible next to a tiny soda can that you can clearly read…
Most game engines operate on the metric system with 1cm being 1unit. Figure out how many pixels you need per cm. Depending on the style and hardware resource requirements of your game you might have to go 1px per 1cm or even 1meter. Those games are typically large blocks of color with very little pixel definition. A good…