haha duh on my part (was equating "i" to inches as "m" to meters)
Hmm, this sets the actual size of the object. I was looking for an additive distance. So typing in 256" on a 512x512 object will make it 512x768. Instead it makes it 512x256.
Actually, Fredo6's Scale shows unit measurements in a tooltip manner, so it's closer to a solution!
http://forums.sketchucation.com/viewtopic.php?f=323&t=17948