I may be wrong but I think 36" is the length, It should fit most hoods for a 36" tank but I would call the manufacturer to be sure or the place where you want to buy it from.
I'm almost positive they aren't actually 36" long. I have found measurements for the 24" and 48" ones (all are less than the labeled size), but haven't been able to find the lengths of the 36" ones.
I imagine it might also depend on what endcaps I use. I was planning on getting the icecap water resistant ones.