Here is how you measure your tortoise http://tortoiseforum.org/thread-6948.html
Hold your tortoise up, facing away from you. Wait till his/her tail is pointed straight down (you can or have someone move the tail if it isn't straight down) than take a picture. This will show how long the tail is. If you can't do that you can measure the tail from the body to the end of the tail.
The carapace is the name for the top shell and the plastron is the name for the bottom shell.