Yeah I mean kids dirt bikes and scooters run 20 somethings on 50ccs.
The magnum X ran a 17, and that was WAY BITD.
For me, tuning control is more important than size. Like I would rather have a mikuni 14 than a sha 16.
I used to love shas, now its like, what the fuck, why can't you be tuned right?
I even prefer the same sized bings to shas now. At least you get a needle then.
Bings are actually more tunable than people give them cred for, ask Roffman.
But to answer the OG question, 18-21 is usually what I rock on kitted bikes. Sometimes a well tuned 14-16.