I need help determining the size of my imac monitor. When I bought it I believe the only sizes available were 21" and 24". I measured my screen at 22.5" diagonally and roughly 19.75" across. This is only the part of the screen that's actually useable. Anyone know which model those numbers correlate to? Is there a serial number somewhere that I can use to look up which model it is? The computer is a little over a year old. It was the first imac with the aluminum casing. Thanks in advance.