You have to wait until the eel is out and about (generally after lights out), then you use a tape measure when it presents the best profile, then you have to add abit of "guesstamation" if the eel isn't perfectly situated. You can likely maneuver the eel with some food on a feeding stick.
You may be able to put a clear tube in the tank of a known length, and the eel may slither thru it, esp. if you bait it...that may help stretch it out some.
That being said, having kept an SFE for 15 years in the past, it will likely reach about 24" or so in captivity. Once they hit about 18"-20" they often get kinda nasty (a sexual maturity thing, I believe)...just a heads up.