What do you mean "they don't do surgery on small dogs"? Please find another vet, this one talks rubbish. The recovery is much much easier for smaller dogs than for giants. You've guessed it - it's the weight. It's much easier to carry small than large body. Also, small dogs can live easier with dysplasia than large dogs, again the weight and body size, so maybe that's why the vet suggested you wait and see.
As for the decision - well, it depends on a degree of dysplasia and your dog's comfort. Injections may help but it's covering up the problem, not solving it. More so, I believe that problem can get even worse, because if she doesn't feel any pain due to injection, she might play too rough or do some move that can further the damage on her hips/joint.
It's up to you really, and the condition of your dog. I would seek second opinion just in case.
Respect and you shall be respected. Animal is always right.