Cody, I agree you definitely should find a phone that you like since you'll use it more than just about any other gadget you own. I never try to talk anyone into one phone over another, so I don't want to discourage you from getting a Diamond or a Touch Pro - some people prefer iphone, some prefer BB, palm etc. They should definitely go with what they want and works for them. If you want to know for sure if the HD is just too big for your needs, just borrow someone's iphone - it is nearly identical in physical size even though the iphone has less than half the resolution of the HTC Touch HD. If the iPhone is too wide or tall for you then you won't want a Touch HD.
I don't think there is a perfect phone - even on the Touch HD - I'd really like a flash since I use them as a flashlight but other than that the Touch HD is the closest thing I've seen to a perfect WM phone in a long time.
The touch HD is about 1 cm bigger in height and width than the diamond but about 15-20% thinner than the US version of the Diamond while still having a 1350mah battery.
You can purchase a number of additional keyboards but here is a video of one of the default ones -
http://in.youtube.com/watch?v=QBO-1D7N4b8&feature=user