When measuring a tortoise, what you want to measure is called the "Straight Carapace Length" or SCL. Following is an example of how to do this. Personally I use a caliper and measure in centimeters. But you can measure in inches as well.
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Put a ruler up against a wall.
put the tortoise on the ruler with the front of the shell against the wall.
Put a board against the back of the tortoise perpendicular to the ruler.
Remove the tortoise and read the measurement. This one is 5 1/8" long.