I'm not sure about the size of the various Windows (PocketPC) based PDA smartphones but my friend has one that is smaller than my Treo 650.
However, I have no problems recommending a Treo 650. If you want something slightly (and I do mean slightly) smaller and less expensive which is also capable of running PlecoDict, you can go with the Treo 600 which is just slightly smaller than the Treo 650 (you will hardly notice the size difference between the 2 units). The negative thing about the Treo 600 is that it is what Palm and Palm marketing people call a "standard resolution device" (160x160) but personally I consider it to be a low resolution device. The Treo 650 is 320x320 and things look amazingly sharp in it. With PlecoDict's magnifying feature, you won't have any issues reading the various Chinese characters even with the Treo 600's standard/low resolution.
I previously used a Treo 600 with PlecoDict and then upgraded it to my current Treo 650. My Treo upgrade was not really because of any problems with PlecoDict since I was very satisfied with how PlecoDict ran on my old Treo 600. I came across a deal for an unlocked new Treo 650 that I couldn't ignore so I bought it (and sold my Treo 600 because the price/interest of it was dropping fast). My Treo 600 was working fine and did everything that I wanted to do. It satisfied all my PDA and phone/GPRS needs.
If price isn't a concern for you between the Treo 600 and Treo 650, I strongly recommend the Treo 650. The Treo 650 does have some negative points but nothing major. Besides the higher resolution screen, the Treo 650 also has bluetooth which the Treo 600 does not have.