Here is a brief yet pretty informative video addressing different methods for accurately measuring the objects you're observing as you paint or draw them. I'm sure even non-artists can remember a time they have seen someone sketching a person, some scenery, or maybe just a few still objects set out in front of them with their pencil and an extended arm. This method is called comparative drawing. In this video, the artist will show you how comparative drawing works. However, as I don't believe in one art technique being the one for everybody, he will follow it with two others, sight-size and triangulation. I'd suggest trying all three of them out in your sketchbook sometime to find out which one comes easiest to you!