It sounds like it could be her first season. I presume otherwise she is ok and is still eating/drinking ok and using her litter tray fine (with no problems).
A female will come into season around 6 months or so and if you've never experienced at cat in heat/season it can look as if the cat is some sort of excruciating pain. They drag themselves over the floor, rubbing their bellies hard against it.They are extremely vocal (with the cry seeming painful and drawn) at this time and will rub and demand attention fuss in the bucket load. They may make 'buzzing' sound as you give them attention and our a lot as well. They will arch their backs when you stoke them and present themselves to whoever is stroking them (their tail slightly to one side as if to accept a male).
I've attached a link and hope it helps you.
If she doesn't seem to fit this description or she's showing any other signs that give you concern don't hesitate to reply back to me and I'll and assist further.
Your vet may make an additional charge if she has been 'caught' and is pregnant at the time they spay . The operation is more complicated and time consuming if she's pregnant and she will need a higher dose of premed /anesthetic as well to produce the same level of anesthesia as if she hadn't been pregnant.
As for her size and allowing her to grow more, sometimes you'll encounter smaller females anyway depending on their parentage. I hope all goes well for you and her.
Hope this helps.