Originally Posted by
ReallyHardcore
If 55cm is my prescribed size, is 56cm acceptable?
Yes (probably). Each bike manufacturer measures and sizes their bikes differently. The best thing to do is get actual measurements of the frame for the seat tube and top tube. At 5'9", 54-57cm seat tube bikes will be in your size range. But you'll have ride them to find out if they actually fit.
If the top tube is too long, you can raise the stem or get a shorter reach stem to decrease the reach. Do the opposite to lengthen the reach.