The first thing i would say is if the size prediction is based purely on the scan and there is no other reason to think the baby is big, then I would be doing some research because these late scans are notoriously unreliable; i know and know of so many people who got told their baby was a certain size (big and small, but particularly big) and they were the complete opposite. So if you don't have any other complications such as gestational diabetes then opting for a c section just on that one estimate is a bit extreme and I would certainly be asking more questions, particularly if this isn't your first baby and you have given birth vaginally before. On Southsides comment re tearing, I had episiostomy with my first due to forceps and tore in the same spot for the 2nd (but ony 2nd degree so I understand her concern totally), did not even feel it at the time and recovered from it very very quickly much quicker than the episio.
Secondly it is pretty rare for women to grow a baby they can't birth. I had a 4.5kg (10lb) baby and birthed no problem, plus i was induced as 10 days overdue so labour was maybe a bit quicker (hard to tell) And I am only 5ft3 and small build. That was my second though, my first was 3.8kg and was a much harder birth in many ways. So all i'm saying is that a big baby does not necessarily equal a difficult birth even though lots of obs in HK seem to suggest it does.
It is your decision obviously, just make sure you have got all the info to make it an informed one.