When you moved the plant to a bigger pot how big did you go? A 4" pot would have been the best size until a larger root system formed.
I think you are right to suspect the cool temperature as the cause of rot. I have had this happen with Hoya imperalis during the winter before.
Hoya bicknellii has very hard succulent leaves when they are mature so I think it will be very capable of rooting without becoming stressed. If you are using an aquarium and can get the day time temperature to get nice and warm then it should root easily. Maybe a light bulb could serve as your source of extra warmth. As for the length I would do whatever works best in the rooting tank. I like to use longer cuttings but in any case cutting off the bare vine above the last set of leaves is perfectly fine. I find this species to be very top heavy and awkward as a cutting so maybe smaller is better in this case, say two sets of leaves per cutting. Are there small leaves developing on the bare vine yet?

Mike

Thanks Mike,

I went from 2.5 in to 4 in pot, it was root bound and tipping over even when placed in ceramic pot for stability. It just got way to tall for that tiny pot with all the vine and hoop to support it. I believe I got it just in time, before foliage got too dehydrated. Will make 4 cuttings from it and hope for the best, if that bare part dies I can always cut it off.
We had very strange weather for south Florida; crazy amount of rain whole summer and now it's very dry with few cold nights in the fifties. It never happens at this time of the year! I guess I need to develop plan for moving my way bigger hoya collection inside if this weather continues! Good part is that some plants got great colors from colder temps ! I had never seen my obscura and nicholsoniae with such great red leaves!
Aggie

I root many cuttings during this time of year because of careless watering.This year I have been lucky so far now that I grow in S/H.
Give your cutting bottom heat with your fish tank and the cuttings should root for you,sooner you cut them up the better your outcome.