9/16" would be for the aforementioned OPC with the 1/2" thread to the crankarm.
a pedal wrench is long but fairly thin, cut from 1/8" steel, some pedals will not let you use a normal 15mm wrench they make many of them much thicker , so pedals in hand you may take them , and your bike, to the bike shop and ask for assistance,
if the 15mm open end wrench you have is too thick and as such, will not fit.