Assuming you have an AT&T iPhone, you can always buy our transparent rear panel for only $30 and take the measurements yourself. I am interested, though, in knowing why you don't already have a rear case of your own to take measurements off of? If you are making it for yourself, I would assume you already have the phone, and if you're trying to mass produce them, I figure that buying an OEM panel would be a necessary cost.
Allows you to show off the inside of your iPhone 4! — 50+ available at 19.95 each.