Last year I bought a Giant Trance SXE+0 Pro MEDIUM from my LBS (they only had the MEDIUM in stock).
They did not have any demo Giant bikes, but did let me ride it up and down the pavement outside the shop.
The shop owner brought in his own SMALL Trance E+2 for me to try against the MEDIUM.
According to the Giant size chart I should be right in the middle of the SMALL frame range (5ft 6in), but to me it felt very short and cramped. When I stood up over the bike I was touching the handlebar and the downtube!
The MEDIUM frame felt much better to me, and gave me much more stand over height, as when I stood up I was further back from the handlebars, and over the lower part of the downtube, and in fact it felt very like my old Stumjumper which was also a MEDIUM.
The moral of this story is that manufacturers' size charts are a VERY rough guide because they work on the overall height of the rider. In reality riders have a great variety of upper body / leg length ratios, which means that the overall height guide is really meaningless.
The only way you can be sure it is the right size is to sit on it and ride it about, even if this is only in the shop carpark or outside on the pavement. It only took me a few seconds of riding each size to confirm that the MEDIUM was the right one for me.
