I'm looking to buy a Cannondale frame so I can build a single speed mountain bike with slicks for the streets of nyc. I'm 6' with average proportions (ie 32" inseam, 6' arm span). Any thoughts as to the proper size? Thanks.

58cm

I commend you for skipping the usual internet forum "advice", insisting to the OP that frame sizing bears little relation to rider height or inseam length...instead, you just answered his question. Now he's happy, the thread can be closed, and we can all get back to drinking beer. Good work.

Note: I'm not saying the usual internet "advice" isn't correct, or more appropriate, and I'm not saying that 58cm is wrong -- or right -- for the OP. I'm just commending you for cutting to the chase.

56cm

58 and 56 are seat tube lengths? I also meant to say a mountain bike frame in the original post.

Probably L, possibly M. Try them both-

I'm a roadie. It shows. Go for a large or a 19" frame.

at 6' with a 32" inseam - MTB sizes - check out a medium, but probably a large C'dale. Hit the LBS with C'dales & test them out before purchasing.

my son was 6'1" with a 34" inseam this past summer, and the L was just right, maybe a bit small - we purchased a XL frame for this season.

Just picked up an older Cannondale rigid MTB in 22" size. It's a bit big for me at 5' 10".
It should correspond with a XL. Their 20" should fit me (L).
I'd say to try 20-22". The skinny slicks will lower the bike too.