I’ve been advised to not breed a small sheltie bitch why ?

Who advised you?  Have u bred before ? Has your dog had all the necessary health tests ( not vet checks) and is this why someone suggested she wasn't suitable?
How small is she as if she isn't a good example of her breed then that could also.be why it has been suggested not to breed.
Could u give people more information  so they can advise

Only the best quality animals should be bred from with as few faults as possible, so if your b itch is undersized for her breed, she's perhaps not typical.   Further, if she is really small, she may have problems whelping.   Have you been back to her hopefully experienced and respected breeder for her advice?   Not all bitches need to experience motherhood.