Space kits do have popular scales.
For unmanned satellites, the popular scale is 1/24.
For manned vehicles, the scale is 1/72 (altho 1/48 and others are around) .
Finally for launch vehicles the scale is 1/144, followed with a few at 1/96 and less at other scales.
You can check out RealSpace Models - www.realspacemodel.com and New Ware - http://mek.kosmo.cz/newware/ . They have a ton of 1/144 scale vehicles.