that'd be a 11" wheel (3.5 sidewall times two is 7, plus 4" for the rim = 11" nominal diameter)
6:1 ratio is golden,
you could get away with 5:1 ratio (in case you do not intend to climb steep inclinations)
And gearing lower than 7:1 might get you into clearance issues with the rear sprockets even with #35 chain so you'd need a jackshaft.
'sid