[OXO cube]

hi guys 

looks like i am delving into another rabbit hole !
and looking into gearing and ratio's 
i understand the basics , but when i decide to buy a gear and pinion set i find there are different diameter pinions and gears especially with regards to anglewinder setups 
i mistakenly thought all gear types would be a set diameter and the same for pinions 
my question is :-
which size pinions to use with which size gears ?

example:-
if i wanted to use a nsr 16.8mm 30 tooth gear with a 10 tooth pinion what size pinion diameter ?
or an nsr 18mm 34 tooth gear with a 10 or 12  tooth pinion ?

why is nothing standardized in this hobby !!!!!!!!!!

kev