As Danny said, 40 IDF's (for VW's) came with 28mm venturis, and there has been 27, 30, 32 and 34's made for them. I believe 44's came with 36's with 28, 30, 32, 34, 38 and 40 mm vents made for them. I think 48 IDF's came with 40's and 36, 38, 41 and 42 mm vents made. As I've said before, there is a 36 IDF- I don't remember what they're off of (a Lancia maybe?). There's usually a set or 2 for sale on Ebay, and I have no idea of the venturis it came with (24? 26? mm), and anything else would be custom machined to order.
Italian 48 IDA's have come with either 37 or 42(I think) mm vents from the factory with 36 to 46 mm sizes available from different sources. There was a baby blue Cal Look Ghia Hot VW's cover car (1977?) with 32 mm IDA vents which the owner had machined up to make the 1600 in the car more streetable. This, of course, was a time when no self respecting Bug-In winner would run anything but IDA's. The smaller venturis apparently helped, but didn't totally solve the car's drivability problems- the carbs were just much too big for the engine.
Dellortos- each carb has venturis a size or 2 bigger than it's Weber counterpart, and what's available for each, sorry but you'll have to look them up. Al
PS- There are 46 IDA's (or are they IDM's?) that came on some later 4 cam (2 liter?) engines. They're pretty hard to find. You'll find 3 barrel 40 and 46(I think?) IDA's on some of the earlier 911's. And Weber also made a 50 IDF- the only thing I know about it is it was on some big English luxury car?