I like the screen size, I don't have huge hands, but they are big enough to operate the phone one handed if necessary (I use two a lot, but I used two a lot on my iPod touch - just the way I prefer using it when browsing).
But honestly, I could go back to a smaller screen if the best phone in two years is a 3.5 incher. I still use the Touch as my PMP when driving, and while it feels strange at first, if the drive is more than 30 minutes, I get used to it by the time I get where I'm going. (Also, I hate iTunes, but I still think Apple has the best music player).
It would feel weird going smaller again, and if all else was equal, I'd prefer a big screen. Of course, if all else wasn't equal, I'd go small if necessary.
But not bigger, don't think I could go past the Evo in size. I think the Dell Streak is past the limit for me. Maybe for women with purses, or men who carry man-bags, but I don't think the Streak will work as a pocket phone.