Barring exotica there are only two 'standard' pedal threads. 1/2" and 9/16"
The smaller is most common on one piece cranks as found on BMX and cheap childrens BSOs.
MOST bikes with separate cranks use the larger size.
As Rob says upthread, remove a pedal from one of your bikes and try it in the crank. If everything is clean the pedal should screw all the way in without need of a wrench. If it locks up after a turn or so there's something wrong. RH pedal clockwise to tighten . . .