I never ran smaller than 17 on the front if I remeber correctly.. I have a 16 from a KO but hesitate to use it if running it hard.
How big can you go probablyat least 80 if not 100.
Dont know why you have 2 threads, told you right off the bat in the other , it uses same chain, only longer and there are many suppliers, just search on your computer.
Failing all that you can go to a bearing or power transmission place, oerder a blank in any size you desire, bore the center, and drill the 4 holes to mount it. When you go real big though you may want aluminum, its a 1/3 the weight of steel.