Two monitors, one iMac, mirroring problem

Here is the background: I want to stay healthy. All this sitting is killing me (and us).

So I attached a 23 inch Apple Cinema HD Display to my 16 GB memory (max) 27 inch iMac Intel Core i5 using a little adaptor. I have one screen up on a tall desk (so I can stand) and one on a lower desk (so I can sit) - this makes no difference to the problem here but, I want to give you a reason why I am asking a question. I use a wired Apple keyboard with the iMac and wireless with the Cinema HD. It all works great. Except for one problem.

I need to mirror the two (not mirroring would not make sense or work as it is essentially just two monitors in different user spots plus 2-3 feet apart on different levels + the menus and windows only stay on one screen for non mirroring - a pain). I only need one screen at a time.

When I mirror, the Cinema HD looks fine but the iMac shrinks down in size. When I choose the 2560 x 1440 iMac size, the iMac just goes back to the smaller 1920 x 1200 and says “Usable resolution 1920 x 1200”.

What is going on and, is there a fix I can employ to regain the lost screen real estate on the iMac screen? The 1920 x 1200 does not look as good on the iMac and the dock icons shrink to be unusable too.