Wow, my current HTC Touch Pro2 feels HUGE compared to the slim HTC Droid Eris. The HTC Droid Eris has a slightly smaller screen and lacks a slide-out QWERTY keyboard, but it's still a great size for a smartphone. In fact, I think that if I were buying a new smartphone today, I'd get the HTC Droid Eris.
On that point, I just ordered an HTC Droid Eris for a family member. It's sad to see that Verizon really hasn't been promoting the Eris very aggressively. The Motorola Droid is a nice phone, but I personally think the HTC Droid Eris is a better phone for my needs. In the past, I used to prefer having a device that included a hardware keyboard. However, as I get more accustomed to touch-screen typing, I'm realizing that I don't need buttons to type.
Although I like my current HTC Touch Pro2, I don't think I'll buy another smartphone that has a slide-out QWERTY keyboard in the future unless the device is really small and thin. A device like the HTC Droid Eris is probably the "optimal" size for an average business smartphone user. My prediction is that over the next year or two, we'll see such a wide variety of smartphone sizes. Some will be quite large with 5" screens and others will have a nice 3.2" screen.