Years ago bicycle frames were built pretty much square. A 54 cm frame measured from the center of the bottom bracket to the top of the seat tube was also close to being 54 cm across the top. This top tube was also level to the ground. Not any more...
  If you have watched any race coverage the last few years, you may have noticed that racers are riding with seat posts way out of their frames and that the top tube slants downward. Welcome to the world of sloping top tube, compact geometry, semi sloping.All different terms for basically the same thing - the top tube is far from level.What is the advantage ? Less tubing cuts weight, tighter rear triangle improves climbing with less flex.
  Now the hard part. At a Trek factory tour some years ago, the Production Manager who was also the tour leader that day, explained that 40% of the frames they sell are 54 and 56 cm. Almost half the market is 5' 7" to 5' 11", weighs about 150-160 lbs and wears a size 9-10 shoe. This is the typical rider profile, a "true medium".So when this cyclist is ready for a new bike after riding a level top tube model for many years, what is the correct compact size?
  To compute the top tube length on a sloping top tube geometry, the only accurate way is to hold a level at the middle of the head tube, swing the level up the seat post till it is exactly horizontal and measure.Orbea bicycles are a bit long like a lot of modern geometries. A 51 cm Orbea has the same top tube measurement as most 54-55 cm frames and is considered the true medium size.
  The following is a reference chart and by no means cut and dry. Everyone is built differently but perhaps this will help based on our years of selling these bicycles.

Up to 5'4"  48 cm
5'5" to 5'10"  51 cm
5'11" to 6' 2"  54 cm
6' 3" to 6'6"  57 cm
NBA Players  60 cm

 

Dama Womens

 under 5' 46 cm

 5'1" -5'6" 49 cm

 5' 7"-6' 53 cm