No tail. Nose-to-tail-base is the standard way to measure. (Or at least that's how my lfs and I do it....)
Put a ruler in the tank... Also, using your hand as a guideline might be good in a pinch...on the average adult human, each joint on your finger is about 1-1.5 inches, which makes your fingers about 3-4 inches each. The average hand is about oh, 8-10 inches when measured across the palm with fingers outstretched, tip of thumb to tip of pinkie.... For the really big guys, Your arm from wrist to elbow, on the inside, is about a foot, and your fingers to your nose (with arm outstretched) is about a yard....
And I thought being a tailor's daughter would never come in handy....
Hey, in middle school we used to take goldfish under the microscope, wrap their gills in damp paper towel, and look at them for almost 10 minutes. They were in fine shape afterwords, none died. Our science teacher actually gave them away after.