Can I use different monitor sizes?

Yes. However, each monitor in an Eyefinity group must display the same resolution - and they will default to the lowest common denominator. Two displays at 1680x1050 and one display at 1920x1200 will all display a 1680x1050 image - stretching that image across the 1920x1200 display. If you utilize different aspect rations (i.e., 2x 1920x1080 and 1x 1920x1200), the image on the "odd display" will either be letterboxed or stretched.