Hi, I can't help you exactly, but I have a 24" Finnex Ray II DS. Without the "feet" attached to each end, it measures 23.75". With the feet, but without the screws that clamp onto the side of the tank, it measures 24.25". With the screws, it's ~24.75".
The cord (when folded flat against it) adds about 1/8" more, but does not add any length when the "feet" or screws are on (sits flush with those).
Thanks. This is very helpful! I probably will lay the light flat on the top of the tank (thus no need for the feet nor the screws). Even with the cord, it sounds like the light will fit within the 36 1/4" hood.
Hopefully Lowe will chime in and confirm the exact dimension. Otherwise I guess I can always take a chance and order directly from Amazon.