40s for road/rally engine and maybe a reasonably tractable 120 bhp (flywheel). Cam, Carbs, Extractors, Compression increase and screwed together right will achieve something like this.

45s for race (for say 160 bhp as stated above but not much happening under 6000rpm).
Obviously needs more work/$ to achieve this.

IN Japan they commonly use 45's on A14-A15,
A10-12 they use 40's..

I would say somewhere between 80 and 160 Horsepower depending on what work you had done to the engine.

I'm no expert. But I think you are asking the right bunch of datsun freaks to answer your question.
I would guess that a pair of 40s would be the absolute largest size you could go for.

wat size side draught webbers would suit an a14and wat sort of power could i expect if tunned correctly