I have a Dell 2707 for my center monitor, and a pair of Asus VW266h for side monitors. That's 27" with 25.5" sides, and all 1920 x 1200. And I feel the same, I'm concentrating on the center monitor, the side ones are just peripheral vision. I don't notice the size difference at all. In fact, my compulsiveness is with the color difference of the 2 brands and different type of panels.
With my setup, the difference is ~1/2 inch on top and ~1/2 inch on the bottom of the monitor. And there was no fricking way I was going to get 2 more Dell 27's just for gaming. Just don't have the deskspace, nor did I want to spend the extra one grand difference.
Anyway here are some pics with my setup. I have since done a little bit of calibrating so the color difference is a little better than shown in the pics. The Asus 25.5 tend to look way too blue out of the box when compared to the Dell.