How do I scale a digital image to the desired physical size (details inside)

My workplace recently changed a new locker shelf. The new shelf is much spacier than the old one, and the door is also transparent. This means that everyone can see what is inside the lockers of everyone.

A friend of mine suggested that I should put up some of my own artwork in the locker as a way to decorate my locker and give students a positive impression :P I think that is a good idea, and I’m thinking of drawing a picture for that, or maybe get a picture I’ve already drawn. The problem is that I don’t know how to scale the picture so that it would fit the space I intend it to. I don’t know how to make sure the picture fit when printed. I’m afraid it will be too small or too big. How can I scale my picture before I print?