No you cannot change the size. The typing is very frustrating at first but after a week it will not be an issue at all. Toying with it in the store is no indication of how typing will be after you've owned it for a week or so.
I text A LOT, and switching from my Curve 8310 to the iPhone was hard at first, but now its not an issue. If I still make a mistake I barely notice because the phone fixes it for me. I think all phones have their own issues and drawbacks but the iPhone is a perfect fit for me.
What most people do not realize is that the iPhone gets easier after using it extensively. You aren't going to be a pro when you first start using it. In fact I wasn't even good at typing on the Pearl at first and ended up loving the SureType more than the QWERTY keyboard. Now I'm in love with texting on the iPhone. I actually type faster than I did on my Curve. My best friend used to say the same and now he has it and he types even faster than me. I type fast too.
I was always a super fast texter with the touch screen qwerty keyboard... i dont know how i do it but god has blessed with a gift of texting fast with any phone hahahaha!