I've had 2 emergency CS, despite desperately wanting vaginal births (DD1 was actually booked for a homebirth).
The recovery is long and at times can be painful (and your tummy won't ever really be the same again). I believe they cut through 3 layers of tissue, and make three layers of stitches. I still get pain in my scar e.g. when sneezing, 2 years after my last CS. There is also the risk of infection and blood clots etc. A friend of mine had a bad infection in her wound, and had an open wound for 3 months which had to be dressed daily by the district nurse. Another friend had an infection which meant she was readmitted to hospital after a couple of days and missed much of the time she should have had bonding with her new baby. Ok, these examples are exceptions, and most people don't get infections, but it is a risk you need to consider. You won't be able to drive for a couple of weeks (this is a big deal for me as DH works away, and we live in a rural village with not much public transport, and no family nearby). So, it is a lot of inconvenience and a lot of pain, and there are risks, so I would not go into it without good reason.
Having said all that, it appears my cervix does not dilate beyond 2-3cm, so I am convinced the operations saved my and my babies' lives, and I will be having an elective CS with this baby, regardless of the risks and consequences of surgery. I, personally, had pretty textbook recoveries from both my EMCSs, so my experience was as good as it could be really.
It can be hard to tell the size while pg - I have known people who were told their babies would be very big / very small but were just average when they were born.
In your position, I would want to be talking to a mw, or asking for a referral to a consultant, to discuss your concerns. You need to be informed about your own risks (ie, does the size of the baby vs. the size of you seem likely to be a problem, do you have any other health issues that could affect matters), as well as the general risks and benefits of both CS and vaginal birth, as this is ultimately a personal choice.
Good luck!