Originally Posted by
revcp
I'm 5'10". My 54cm 1988 Miyata Twelve Hundred fits perfectly, as did the Miyata Team and 312 of the same size that I've had. A 56cm 210 and Six Ten were both too large (I sold the former, built up the latter for my 6'1" son). My guess is a 54cm is the correct size, but it may feel different from what the OP has been riding.
54cm is probably a bit small for an "average" 5' 10" male. But, there is a large range of acceptable fit, especially if you go to extremes like extra long stems. For example, I'm 5' 11", but with ridiculously long legs (35.25" cycling inseam). I need tall frames, with short top tubes. My small bike is a 57cm, while my most comfortable fits are on 62-65cm bikes, as long as the top tube stays under 58cm or so. If the OP could measure his cycling inseam, that would tell us a lot more about his "ideal" fit. That, and his intended riding style. Flat-out racing favors the smallest bike you can make fit -- both for weight and aero reasons. Casual, around town type riding favors the other end of the spectrum -- large frames to get the handle bars up higher for comfort at lower output levels.
Older bikes tended to only vary the seat tube -- i.e. a 52cm and a 62cm bike might both have a 57cm top tube. It wasn't until the mid 80s that "proportional sizing" became more common, and you started seeing things like 59cm top tubes on 60cm bikes.
I agree with some of the earlier posts, I would look for something that fit a little better first. It will take the same amount of time and money to overhaul the right size bike. If you're handy with tools and mechanical things, you're still looking at about $50 in tools, plus $50-80 in tires, pads and cables to do it right, or up to several hundred to have a bike shop do it for you. That's an expense that you won't get back if you decide it doesn't really fit down the road. Putting $100 into a $100 bike doesn't make it sell for $200, It's still a $100 bike to most folks.