For the short answer, they can be anywhere from 8-12", so it's not a real accurate average. Depending on their genetics + how much + how often they are fed . When I was breeding them I had some 3 month olds that were over 14", but the average is a bit smaller. The common way to measure is nose to tail tip. I general, no one posts that their dragon is 10" and excludes the tail purposely, which would make their dragon actually in the range of 19-20". We owners/breeders include the tail length in the actual measurement. We would also take in to consideration if the tail was nipped which could make the over all length shorter.
How big is your dragon, do you have any pics ?