I REALLY doubt there will be 2 sizes of iPhones without a SUBSTANTIAL difference of quality. I really don't think the addition of gps would alter the size so dramatically.
I couldn't imagine apple making 2 sizes of iphones anyways (without making one an uber tiny nano-like phone) so I don't think that'll happen. But IF it does the big one will have gps, and video chat, and better camera, and perhaps even more (exclusivly 3g?), leaving a thin basic version of the iphone for $200.
Too many rumors! But I am 95% sure they will all be the same size. It'd be much simpler to make cases and cause less overall confusion. Apple is all about keeping things simple for the consumer.
I agree, two different sizes would be too confusing for the typical apple consumer. Just let it be one size fits all!