Your poll isn't differentiated enough. 12+ is too much of an open ended choice.
My favorite size is 10-12 bosses. Its long enough to allow meaningful progression yet not that long that you can't clear it in one 3h session during farm. Most of my favourite raids had tiose numbers of bosses (Ulduar, ICC, ToT).
Also, having a choice is preferred, like in Ulduar (lesser in ICC) and NH. EN was too short. Naxx had choice, but was too long. It was tiresome.
3 bosses is an absolute joke of a raid and is only acceptable as a filler like ToV.
6-8 is acceptable if there is two raids of that length in the tier (think BWD / BoT / Throne in early Cata).