Stupid Question Day: What Kind of Squash is This?

Okay, I am going to admit right upfront that I feel incredibly stupid for asking this question.  After all, I bought the little squash plant from a nursery and intentionally planted it in my veggie garden:

A couple of weeks after transplanting

I have lovingly watched over it and cared for it for weeks, as it grew into this monster:

I protected it from leaf-eating groundhogs:

I've watched the squashes grow from little tiny acorn-shaped nuggets into this:

But, I'm not actually sure what type of squash it is.  You see, I just bought the plant on impulse, along with the zucchini plant, while I was at the nursery.  I wasn't even planning to grow squash this year.  I didn't bother to ask what type of squash it was, nor do I recall it coming with any descriptive information.  I've run some google image searches for different types of summer squash, but haven't found anything that looks exactly like this.  The closest I could find is a round zucchini squash, but these look lighter and more "scalloped" than the other images I've seen.  Does anyone know what type of squash is growing in my garden?  Also, how big should it be when I pick it?

Thanks!