They spayed at 8 weeks?! Not saying that's wrong, just I've never heard of it.
My Emma has an umbilical hernia too. It's gotten a little bigger as she's grown, to a little larger than a quarter on an 18 lb pup, but it's not causing any trouble. Per the vet's suggestion we're waiting to have it fixed when she's in to get spayed.
Since you don't want to put your girl through anything so soon, I'd think you can wait. I don't think it will close on its own, though.
I was concerned about Emma's until the vet let me know that if it ever starts bugging Emma, she'll start to lick at it and then we'll know to take her in right away. Until that happens or she gets it fixed at 6 months old, it's just a part of her anatomy and nothing to worry about.