Hi Alex,
I'd reduce her food intake during her convalesce (sp?). No exercise means easy weight gain. Her muscles will be poochy until she heals up. Then she'll probably look as she usually does. As for feeding her, well, don't worry about it unless you notice her putting on weight.
Just between my mother & I, we've had over 12 spayed bitches of various breeds during the last 20 or so years. Recently, I've known other friends who had some too. None ever gained weight just because they were spayed. It usually comes with inactivity..... not enough exercise + more food than they need. Sometimes their metabolism slowed, but that was usually age or breed related. Samma had a very slow metabolism before & after her spay. That dog puts on weight just looking at food :D
Hopes this eases your fears :D
toodles