Singlespeed & Fixed Gear - Frame size question

I've got a question about frame size. I have a very nice Marinoni that I want to convert to my first fixed gear. The only problem is I think it's kind of big for me as the seat is no more than an inch above the handlebars and probably closer to a half inch. Is the frame simply too big for me and does it really matter anyway? What are the actual disadvantages to having a frame that's a bit too large for you anyway?

I'm 5'11 and I think the frame is a 56, which should be fine but I've got really short legs and a long torso. The frame feels great to me, but it's the only road bike I've ever been on and really don't know how it's supposed to feel, or if my performance is suffering because of it.

Thanks for the help guys.

I may be ostracized (sp?) for saying that, but I think you can make do with a slightly mis-sized frame pretty well.
It's hard to tell without seeing you in person on the bike, of course. But if you have a long torso, then the longer top tube of the largish frame should fit you fine. If you have no standover issue and don't mind the look of the bike with only little seatpost showing, then the only possible problem remaining is the handlebar height. You might be able to address that with a new stem, or leave it as it is: a high bar wil be more comfy than sporty or aero, but if that's fine with you...

BTW, 56 for a 5'11 guy should be OK, shouldn't it?

I must link an Hungarian public education video that advises against (among other things) using a too large frame. It's the first on the page titled "Kerékpáros oktatófilm a 70-es évekből" Funny as hell. Enjoy.

http://www.alleycat.hu/?q=taxonomy/term/27