Batting can vary based on what it is made of and if it, was, or requires pre-washing. I suggest you always read the package directions. If pre-washing is required, you can expect cotton batting to shrink up to 5%. Polyester batting will have least shrinkage. Think about whether your batting will be large enough after prewashing it. When in doubt, buy a larger size.
If you buy quilt batting off the bolt, you won't have folds. But if you buy pre-packaged batting, you will. Here's how to get rid of those pesky folds: Remove batting from the package and gently unfold it. Toss it in the dryer on a NO-HEAT setting for a few minutes. What a difference this makes!