I don't think anybody has an official number for you. I'd say just go with the length of a fluorescent light tube, since that's obviously what it is. ;)
"The worst part about these reviews is they are [subjective]--and their scores often depend on how drunk you got the media at a Street Fighter event." — Mona Hamilton, Capcom Senior VP of Marketing
*Image indefinitely borrowed from BrainBoxLtd without his consent.