When you create your character, select the option to save and it will show you the weight on the screen where you give him his name/nationality/etc.
It can take a bit of trial and error (which is the reason why there is a range, and not an exact number), but not much.
The reason for the weight restrictions is that it was one aspect which people felt was unrealistic about some of the 40-90 characters, and was the subject of a lot of discussions. It's debatable on whether or not it really has an effect, but for the sake of putting the arguments to rest, they are part of what is restricted.