Knowing what size bike you need is tricky, even if you are actually sitting on the bike it can still be a bit of a mystery unless you have a Bike Doctor to help you.
Below is a basic guide to help you choose the size of bike that you will need. The reason sizing is tricky is because of the differing frame geometries between different manufacturers, take that and combine it with our different body shapes, it can get a little tricky. An example of this is that Devinci bicycles, as a rule of thumb, have a shorter top tube (the tube that runs horizontally between the handle bars and seat post) than Norco bicycles. The result of this when trying a similar bike from both manufacturers in a medium size for example, is that they will both fit you differently. The Devinci with it's shorter top tube will generally fit someone better who has long legs and a shorter torso. The Norco on the other hand, with it's longer top tube will better suite someone with shorter legs but a longer torso, and with longer arms.
Of course this is just a basic guide, not a definitive guide to getting the perfect size bike.