I am rather new to this as well....someone will correct me if I say something wrong....
You won't be able to tell if she is pregnant for at least 4 weeks...if not 5. Your vet can feel for "bumps" at that time but an xray (there is a good week for this which I have yet to learn of) at which he should be able to tell you how many are in there. At some point he should be able to give you advice on whether or not she can deliver them based on size. The weight difference doesn't seem like alot so I don't know if that's a huge issue.....but one of the long time breeders should be able to give opinion on that.
THere is ALOT of reading to do in preparing for pregnancy and whelping (delivery). You will need to ask alot of questions in order to feel prepared.
I hope all goes well for you and your dog. Don't be too hard on yourself....accidents do happen. And...if she doesn't end up pregnant and you don't want puppies....book her an appt at the vet
Good luck!!!