We've only got 120mm x 60mm to work with... and I was thinking of keeping something along the lines of a Drawer Desktop File Clear and A4 Document Box, in addition to an iMac on the desk.
Going with a larger iMac will obviously leave us with slightly less room on either side (12.2cm all up) ... but does it really matter? Is 12.2cm worth crying over? Further down are some diagrams of how much difference there is between the two. MrB obviously wants a bigger screen... and although I would love a bigger screen if someone else was paying, I am thinking the smaller one will do the job...