As Maelochs said, probably something like a 58. But, there are other dimensions that need to be considered, such as inseam and reach. If you are buying from an LBS try to find one that will give you a basic fit before choosing a bike. If you're buying on-line that Competitive Cyclist site is good. Be aware that you can probably fit in a range of sizes. I am 5'9" (thanks to age) with a 32" inseam and I can ride from 54-58. Of course stem length changes with the extreme sizes. Back in the day you would measure your inseam wearing only socks, convert to cm and multiply that number by .667 and you'd be in the ballpark. Different these day due to sloping TT.