I'm running two 7" USB monitors and the size issue really isn't that big of a deal to me. It took some playing around to get the MFCD sized right, but it works great now. The major issue is one of the monitors will not refresh properly when the game first loads, but cycling the power resolves the issue. It will creep back up though, either from alt-tabbing or over time. A power recycle always fixes that though.
I also bought a 7" touchscreen to use with Helios to make a CDU, but the touchscreen is so horribly inaccurate it's unusable. There is no reason I can think of to use touchscreens for MFCDs, so you can save a little bit of money there.
Oh, if only I could find a cost effective solution for the CDU. :(