Try not to worry too much. First, if you’re going to a rabbit saavy vet, then a spay is one of the most common procedures, and actually the bigger the bunny, the better. Some vets wait until a small bunny gets more grown to be able to do the procedure due to size, so a Flemmie is a nice sized bunny to do at any age.
You should make a list of questions to bring, and ask the vet each one at the time. The vet is supposed to give you instructions, meds, and show you how to use anything you don’t know about. Just speak up. Take notes. And do not hesitate to call your vet with questions after your bunny gets home. A spay is a process involving you, too, and they are there to do follow up as well as the spay.
Don’t be worried if your bunny has little appetite for the first 24 hours, either. No jumping, keep everything on ground level, no stairs. If you want to see if the incision site is healing well (watch for excessive redness, swelling or oozing, which doesn’t happen often) then place a hand mirror on the ground under her feet and dangle a treat above her nose so she lifts herself up a bit to get it. Look in the mirror to see under her. It’s much easier for you and better for her than flipping her over to look. But don’t worry about doing that the first 24 hours either. You’re making her life much better by doing this! 