try and find something on craigslist. 90's cannondales are really nice bikes for the money. as far as components go on the used bikes, look for shimano 105 or ultegra. anything that says "tiagra" or "sora" is shit. if you're really going to ride the bike a decent amount the extra $100 or so for the better components and probably wheels is SO worth it.
as one guy suggested bikesdirect.com is a great site and is 100% legit. the only catch is that you need to know how to assemble a bike, not just screw stuff together, but know how to adjust a headset, bottom bracket, true wheels, everything. I'm sure a community shop could help you out. again, stay with shimano 105 or better.
as far as size goes, you're going to want to shoot for a 57 or 58. it does depends on the person, everyone's different, but most 56's are going to be too small. Also, there's no guarantees with size because each brand measures differently and have different top tube/seat tube lengths. this is why going to a shop to get sized just to buy used is a waste of time.