I think a 56 would be just about right for a 'normal' 5'11" guy. Sure the geometry of the bike will be a player, but in general terms 56cm is just about right.

The last Craig's list bike you posted is about a 62cm bike. Wayyyyyyy to big, and a POS at that. Don't be so eager to buy the cheapest thing you can find.

The Fuji deal that is being offered to you is a really good one. You could ride that bike for years, it needs no work, it has a modern drivetrain, and it will actually have a little bit of a resale value after you put 20k miles on it. If you don't have enough for it, save some money up. I know that might mean actual labor and not instant gratification, but you'll get a lot better bike in the long run.