If you got her from a pet store, there's basically no chance of her being a giant.
I definitely wouldn't breed VTs. Even if you're not *planning* on getting 100 or more babies, you might get that many anyway. Unless you're going to cull, you can't really know how many you're going to get. Unless your LFS is a really good one, it probably isn't a good idea to give them your bettas. I know I wouldn't ever trust a pet store enough to give them my babies, and you never know who will buy them. Also, you never know how old pet store bettas are. Most of them are at or past their breeding prime when you buy them.
You're best off just waiting until you can get a breeding-quality pair. You can usually find some for pretty darn cheap, and it will definitely pay off when you've got a bunch of betta kids to find homes for.