How do you you measure your arm to fit a viola?

What you need to do is go to a store that sells violins. The person selling you the instrument will ask you to hold out your arm and put a violin under your chin like how you would usually hold it while playing. You then wrap your hand around the scroll (the swirled wooden end of the violin with the tuning pegs) and if it fits right in your palm, the violin is the right size.