I cannot have full screen when recording with 550D (T2i) and lilliput 8" 869GL monitor.
When lilliput set to 16x9 it's squeezed when recording (good when not recording)
When lilliput set to 4x3 aspect ration is good, but image is sized down when recording (when not recording it's not good at all)

This is a problem with what the t2i outputs. It sends a letterboxed 4:3 signal out. The monitor is displaying what it is being sent. The only real "solution" to this is a monitor thats designed specifically to compensate the annoying signal sent by the camera.

Your monitor is doing what should be expected with the funky output from the camera.