The septum of your nose is located deep inside your nostrils. Therefore, a procedure to straighten your septum should have absolutely no effect on the size of your nostrils. I would not worry about the cosmetic effect of the septoplasty, as it should have no discernible effect on your external appearance!
You will need to talk with your ear, nose, and throat surgeon
specifically about the details of your case in order to get answers to the rest of your questions. Every case is different, and only the surgeon who is going to perform the procedure can answer your questions.
I would recommend that you make an appointment prior to the surgery
with the ear, nose, and throat surgeon to get all of your questions answered and your concerns addressed. Before you go to the appointment, write down all of your questions on a piece of paper. This will ensure that, when you go to the appointment, you don't forget to ask them all.
Generally speaking a septoplasty is a very simple surgical procedure and it is well tolerated by most people without much pain. Additionally, it will help with snoring
, nasal congestion, and similar symptoms down the road!