correct info above

there are two standard sizes of pedals you are likely to see
one is 1/2"
which is what you use on your old bmx
because it is the size used on one piece cranks
and everything else uses 9/16"

so if your diamondback does not have a one piece crank
it uses 9/16" pedal threads
and that is the size that all clipless pedals come with
with very few exceptions
in fact
you would have to search really hard
to find a set of clipless pedals that don't have 9/16" threads

all this is to say
you are probably overthinking this

go to the lbs and every clipless pedal they sell
will fit on your diamondback