PS--Your girlfriend probably just isn't used to large things.
LMAO!!
Personally, I think the 20" iMac is too small, both from the perspective of visual real estate and from an engineering perspective - it's too much of an enclosed space for hot components inside.
I was, up until one week ago today, using a 28" monitor on my Macbook, which was great for watching TV shows and films from iTunes, but as something I had to work with every day, I found my eyes darting to different parts of the screen because they couldn't take in the sheer size of the screen all at once. Now, whilst this is, in part, due to the fact that my desk here at university is not as large as my desk at home, I believe it's partly because 24" is the sweet spot for display sizes.
This is because 24" monitors support a resolution of 1920x1200, which is the highest resolution screens can achieve under 30". So all those 28" TFT LCDs on the market can only beef out a resolution equal to a 24" screen, and due to the increased size (4" diagonally is a lot) the picture is actually of a lower quality. You also have the added benefit of being able to take in the beautiful vista of your desktop without having to move your eyes to different parts of the screen, which is so much of an annoyance
EDIT: to reference the old adage - all size is relative. Whilst the 24" will seem huge at first (heck, it's 9" more real estate than you're used to), you will acclimatise to it quickly. As a point of reference, the 52" LCD TV in our living room seemed gigantic when we switched from our old 32" Plasma TV, now it feels so small we're considering a high definition projector to use the entire wall as a TV screen.