You don't ask for much do you? That's a pretty broad question - maybe it would be easier if you defined what it is you're trying to do? Fuel? Oil?
AN fittings are a size standard, aircraft and hydraulics are the two areas I've seen them most. The reason why this is a tough question is because of the fittings - straight AN -6 might flow X amount but put it through 4 fittings indluding a couple of 90degree units and your flow will drop. Pressure that you're flowing at might also make a BIG difference but I'm NOT sure. 90degree fittings are supposed to flow poorly and tigher bends just get worse. Even straight fittings have some flow loss. Special fittings for pumps that are radiused are sold to provide some relief at the pump from this.
For fuel systems I'm used to seeing -8 to the rail (V8s) with some of the big dogs running -10 or multiple lines. -10 is what I've used to the pump from the sump usually. I used -10 ONCE for a feed line from the pump - had to turn the key twice to fill the line to pressure with a stock Ford ECU. I have -6 on an oil feed for a V8 that I think might actually be too small, I'm going to port some of th efittings first before I move up though.
I've noticed that bypassing the damper thing on the Supra gives -6 but that seems a little small to me for big numbers. -8 is what I'll end up running I'm betting unless someone can provide numbers to support using -10. Higher the -# the larger the diameter. Hunt around here and you'll see several people's setups for fuel inclusing some I think that run dual feeds all the way from the pump(s) to the rail. Lot's of options it seems
The weirdest thing I think I saw noted was a -10 oil drain. Seemed big to me but I'm told that oil gets aerated pretty good in the turbo bearing so perhaps this is to get the mousse to flow better? Anyone? Still learning about Supras here