I can't sew to save myself, so I just bought them! As far as I know they come in a standard size, I've never seen different sizes. But I could be wrong. Anyway, as long as they're square you shouldn't have any trouble, just find a fold that suits you. And some covers!

Good luck, I'm sure you'll love the baby beehinds (most of us do!) but it's good to pad your stash out with flats until you know what you like, and can afford it.