When it comes to blankets, one size does not fit all. Especially when it comes to baby blankets. I have found a few sites that make it very easy to know what size blanket that you will need to do.
This is a very good site for reference for almost anything crochet! http://thecrochetcrowd.com/afghan-sizes/
I recently needed to figure out the perfect size for a car seat cover and this site helped me with that http://www.goodknitkisses.com/Resources/blanket-sizes-chart/
I usually tend to make it a little bigger then what it suggests. That way you know it will be the perfect size!
Another thing that needs the perfect size are hats. There are many sites that give you measurements on circumferences. I like to just try it out on someone instead! (However, if you don't have anyone to try it out on, you might want to follow the measurement on the pattern.)
For newborns and babies, I just measure it up against one of my son's old hats. However, I have made so many of those that I can usually pretty much get it right just by looking at it.
Then, for small children, I have my son that can help me out with that! He isn't always very fond of trying things on for me, but I can usually get him to do anything for chocolate :)
For everyone else, I just try it on for myself. For women, it should fit me. For men, it should just be a tad bigger on me.
Just remember, before you start a project, make sure you know what size you need it to be. You don't want to have to frog it in the middle and start over!