On their website there is the official dimensions but in hand it is roughly the same size. The plus is ever so slightly thicker in the middle when laid next to each other. In the hand they feel the same since the backs are curved like the iPhone 3GS. In reality that is how it feels which is the reason I like the Mophie over the other battery cases.
The Plus weighs slightly more on paper but when comparing both an air in one hand and a plus in the other, they feel about the same. The difference is minimal and to me not noticeable. After a day of use anyway, it feels on par with using the phone with an Otterbox in terms of weight.
I prefer the Plus for the reasons above and just the simple fact that it feels more solid to me. I have a bunch of battery cases. I have, off the top of my head, a Powerskin, Phone Suit, some generic one from eBay, a Case-mate piece of crap, and the offGrid from Incipio. The Mophie Plus is the best of the bunch in terms of a trade off of feel, weight, and overall function in my opinion.
It isn't the thinnest/lightest of the bunch (that is the offGrid) but it will and has survived some rough stuff and just feels good in the hand. It feels like a slightly bigger 3GS and to me that is fine. I personally hate the iPhone 4 naked.