Yep, a metric ruler will do for the most part. You just look at the most unsquished part or pick a pad size that is thicker then the compressed size.
You could probably re-use the pads. However, some OEMs use a fibrous gummy pad that basically disintegrates when removed. Some times they use a thermal paste like goop that hardens (haven't seen that in a while)
If they use the more spongy type pads, they can indeed be re-used. It isn't ideal, but better than nothing.
Really you should only re-paste and re-pad if you are having temperature problems. Though I can understand some general maintenance at this time to ensure your card stays in working order.
There is some risk in taking a GPU apart. The GPU die is exposed and if you damage it, well , end of card. You could potentially damage a surface mount component when removing the heatsink. Or even rip out a fan cable or something. Not a big deal as long as you are careful.