I don't know if you work relative size or sight size.
Sight size is a great way to do figures, portraits etc. You place your canvas next to your subject. It seems a bit awkward at fist, but you can stand back and compare the painting to the subject. You can measure carefully with this method also.
Here is a thread discussing it. There are more. Just put 'Sight Size" in this sites search engine.
I find it a very convenient way to work. In school with a lot of other students it may not be possible tho'.
http://forum.portraitartist.com/show...ght=Sight+Size