2 stroke? It totally depends on how much power your making which depends on many many other Things like state of tune altitude and snow conditions. If you have deep deep deep snow the best gearing is to be kinda high so it almost wont pull 2nd unless you tap it way out in first to get there. Those super deep days If you are able to pull 3rd you are probably geared to low.
the best way to judge gearing on a snow bike is to"
see how far your track runs when your counter shaft sprocket on your bike makes on full turn. So mark the sprocket with some paint or sharpie marker, mark your track and rail, turn the sprocket one full turn, your track should go 13 to 14 inches. My yz 250 two stroke did best around 13.5 inches, it was a hand make kit, my ktm 500 with TS kit 120 rolls out to 14.4 inches and I like it there.