Very good question
I would guess that, for me (and others), it's a case-by-case thing, and sometimes a subconscious choice
Yes, size, age, apparent (toughness) is a major factor, I mean, to use my usual example regarding this aspect, Chyna (in her prime) vs. Callista Flockhart (or lesser margin case) would be a no brainer
... then again, I do also like an ''upset'' here an there, Chyna, Serena Williams ect... ''should'' lose one now then, be it due to said's over-confidence, or just lucky shot or move for their opponent
Like or dislike ?
As far as the women that I like, I mean, my top favorites, it's almost never a factor for me, as I enjoy ''seeing'' them being triumphant, as much as I do their winding up being the victim
As far as the women that I dislike, I mean, the handfull that I plain just can't stand (Spears, Hilton, Harding, Kardashians, ''Jersey Shore'' girls and alike), it is totally a factor for me ! I would only want to see any of those in a two-second-long, one shot, k.o... only for the fact that I DO so dislike them, that I refuse to even consider having them even that kind of a bout !