All of my snakes are in constant motion, even the BP and Boas. Every time I reach for the tape measure they start squirming.
I can get a very close estimate when they stretch out along the length of the enclosures.
I got an almost exact measurement on the boa yesterday. 6' 3". I'm suspect he'll have a little spurt of growth when he resumes his spring/summer feeding schedule, but I'm thinking he's about to slow to maybe an inch or two a year from here on out.