It really doesn't matter how large your thumbs/fingers are when you "get" how to type on the iPhone keyboard. With even the largest thumbs when typing using the edge of a thumb the actual contact patch (when taping with a gentle stroke) is still smaller than the keys on the iPhone portrait mode keyboard.

It helps when first using the iPhone keyboard to know about the rocking you can do to correct errors before they even occur. Most people assume that when the letter pops up that is the one selected and release the key even if they got the wrong letter. It makes it much easier to get the hang of if someone points out that if the wrong letter pops up you just hold the tap and rock your finger slightly in the direction of the right key (or slide) and then release when the popped up letter is correct.

I far prefer the portrait keyboard to the landscape one. The landscape one just takes up too much space and I understand completely why most app writers don't go to the trouble of supporting it.

One thing the iPhone really needs is a switch to disable screen rotation. It is a total pain when laying in bed reading/typing and the rotation keys kicking in.