Firstly I'd get a collar that fits your puppy now, as he grows you can get a larger collar. Jett is onto her 2nd collar and she is 5 months old (picked it up today).
Collars today are adjustable so you should find that when he if fully grown an XL (Mastif/Rotty) size 'should' fit. With regard to correction collars, prongs etc, you can get them from your dog trainer. They will suggest what is the right collar for your dog.
Is this your first rotty, if so, welcome to the wonderful world of rotts
You will need to start training your boy and socializing him asap. These dogs require lots of mental and physical stumulation. I'd enrol him now into the next puppy kindergarten (your local vet may have one beginning when you pick him up).
Yes his parents are out of standard in height so showing him 'may' not be an option, but that is not the be all and end all. As long as you love him and train him he will be a friend for life.
Just be wary of the weight factor....these dogs are prone to hip dysplasia and acl tears and weight does come into it. Your vet will advise you of the weight you should aim for.
Make sure that you give him a good quality puppy food so that he doesn't grow too fast, you dont want to strain his bone development.
Anyway, have fun picking him up and get a collar that will suit him now, like i said earlier, you can get a new one when its required.
Good luck!
Trina