Hi, Norine, glad to see you started your own thread. You really need to have that x-ray done to see the # and size of the pups. That will go a long way in helping you know what you are facing.
Though x-rays are often off on the number of pups, you will get a good idea of the size of them and will know if a c-section is likely due to size. Of course, that isn't the only reason for a section but is info you need to have.
Yes, the dam controls the size of the pups in utero. Having more pups generally will keep the size down a bit as opposed to a single pup, but again, that is not always true. You can have a big pup in a large litter or a small single baby. The only time one of mine delivered a singleton, it was not a big pup. Nothing is 'cut and dried' with litters and every one is an individual event.
There are complications that arise beyond the size of the pups. Usually about half of them will be head first and half butt first. There are other presentations and some are very difficult for the Mom to deliver. You will need to familiarize yourself with these so you won't be caught off guard.
Have you checked the link given in the other thread? If not, please do. It's full of good info and includes many whelping pictures. Here it is for you again......
Dog Breeding and Canine Reproduction by Debbie Jensen. Dog Breeding and Whelping guide for dog breeders.
Get your girl in for that x-ray and you'll know much more. Make sure you have a whelping area in a quiet part of the house set up and get your supplies together. Remember that it's not unusual for them to deliver early so don't count on a fixed date. Keep us posted and ask any questions you may have.