It really is important to have her spayed as soon she's ready. With every heat, their chance of developing mammary cancer increases. The risk in dogs spayed prior to their first heat is 0.05%. It is 8% for dog spayed after one heat, and 26% in dogs spayed after their second heat.(
Spaying (Ovariohysterectomy) Female Dogs) Doing so will slow down her metabolism but it shouldn't affect her size too much as long as you don't overfeed her and make sure she's active. That's the case with any dog though, spayed or not. I've never heard anything else about it directly causing her to grow larger, though.
I hated the one heat we had to go through with Maggie. I'll never do it again!
