If you bought the 25.5" Asus and used it as your middle screen, you probably wouldn't even notice the difference. You can set bezel compensation to deal with it anyway.
27" will probably be fine too, but if possible I'd definitely try to get the same screen you have already. You do use the side screens more for immersion factor and it's very unlikely you'll notice an inch or two diagonally.
My Lenovo T61p has an older LCD in it and my new work Lenovo W510 laptop has LED. I notice eye fatigue is worst for me for some reason on my new LED laptop unless I critically lower the brightness and it just doesn't look as good too me so I think I am eliminating the LED version. Maybe I am wrong though and LED has nothing to do with my eye fatigue. I do need new glasses.