I'm a guy, so maybe some ladies will tell me if I'm wrong, but it seems to me that in a threesome the face won't factor in but the penis size will. I would think that her needs come foremost, and therefore the average size guy should do her doggy style first, and that leaves the big guy being given a blowjob first. My reasoning is this: It seems to me that for it to be a fair and balanced threesome both guys should wind up doing her doggy and both guys should wind up getting a blowjob. Whether she can suck on the bigger dick easily isn't a factor, as it's not going to change the level of sucking difficulty whether he goes first or second. However, penis size may factor in in regards to her vagina. If the average size guy goes first it can act as a warmup for her vagina to take the big one next. But if the big one goes first she may not be as prepared, plus she may not can feel the average size one as much when it comes his turn to do her doggy.
I recently watched an amateur porn video in which one guy had a smallish penis and the other guy had a biggish penis. There was quite a difference in their penis sizes. The smallish guy went first with vaginal intercourse whilest she sucked the bigger dick. The smallish penis actually was able to get some vocal from her at first, but apparently her vagina expanded some more and then she became silent. Eventually the guy with the biggish penis took his turn in her vagina and she became vocal again, not only did she become vocal again her level of vocals were far more pronounced. She didn't spend any time looking at their faces.
So to me, in summary, logically it should be this way:
1)Guy with average size goes first doggy.
2)Guy with big dick goes second doggy.