Over the years with my own rabbits and with the rabbits at the sanctuary where I volunteer, I have seen pairings of rabbits of sometimes vastly different sizes and there was never any problem. My two current rabbits are different in size, with the female being about twice as big as the male, and they are fine together.
Regarding clipping Stella’s long fur, I think you will have to wait a little while on that. Since you just got her, it will likely take her a while before she lets you clip her fur. You really will have to be careful with the scissors (if that’s what you are using) on her belly area. When she goes in for her surgery, which I guess will be in a couple of months, they will of course shave her belly then. Hopefully, she will be okay with the long fur until then, and afterwards when it grows back, maybe she will be comfortable enough with you clipping it when needed by that time. While she is under anesthesia, maybe they could give her a general trimming all over or where needed.
I once adopted a long haired rabbit that came to us with very matted fur and I accidentally cut his skin when trying to get the mats out, so I’m always wary about using scissors on rabbits. Dog groomers aren’t usually good with rabbits either, and don’t know how to correctly handle them, so make sure that you keep Stella well groomed so that she doesn’t form any mats.