Hi Lulu, I’ll try to answer some of your questions the best I can.
As for size, it really doesn’t matter. Many bunnies bond just fine with big size differences!
As for bonding, it’s really going to depend on the two bunnies. You will want to start small, like with the bathtub and slowly work your way up to more space. Don’t rush it. We did the bathtub 3 or 4 times per day for a week or two before we expanded. Some bunnies were just meant to be together. Mine got along quickly but would have never bonded in one day even with a marathon session.
It is definitely possible that two bunnies might never bond and you should be prepared for that reality before you bring the new bunny home (by having a separate space to house the new bunny). You might see if your bunny can meet the new bunny before you make the commitment (but not if you think the new bunny might be sick).
You also might want to reconsider getting a second bunny if you know you can’t afford the vet bills. It is inevitable that you’ll need the vet at some point and time (on top of needing the spay). You might want to look into pet insurance or something like that. If you do bring the new bunny home you should house her separately until she gets fully checked by a vet to make sure she can’t spread anything to your guy.
I’m glad you want to save this girl–it sounds like a pretty awful situation and you would give her the best care possible! Good luck on what you decide!