RE: Welcome to Club SAITO !
TC:
I've always considered 4" to be about the best length for the breather hose. The problem is that the hose itself restricts the flow, and the longer the hose the higher the crank case pressure will run.
Since the lubrication of the bottom end of the engine depends on ring leakage the higher crank case pressure will naturally decrease the amount of oil coming past the piston ring.
As a practical matter I've not seen any problem with the vent hose up to about 12" length, but if I go longer I'd use the large size hose to decrease the case pressure rise.
Bill.