Tsclient has been hard coded for a series of resolutions. While this may have made sense in the past it is far less useful now.
Almost all of the hard coded resolutions are based on a 4:3 aspect ratio instead of a widescreen 16:9 ratio that is becoming more common on laptops.
Also far more common is people having multiple dislays.
I, and I am sure many others, would like to be able to use 90-95% of my wide-screen laptop. I would like to have a desktop span the width of two monitors without having to go go full screen. rdesktop supports this just fine if you pass the correct parameters.
This should be easily accomplished by adding another item on the 'Display tab' under the 'Remote Desktop Size'. I think you would want to add a fourth radio box with text like 'Manually specified screen size', and then two text boxes for the horizontal and vertical resolutions.