terminal services client screen size a %

Hi. How do I change the windows terminal services client screen size to a percent?
Right now the only choices are set sizes such as 800×600 and 1024×768 or full screen. The set size windows don’t work because many users have wide Excel spreadsheets they need to see. Full screen isn’t good because some users have programs installed locally they need to use and so then they have to keep flipping back to their local desktop. Right now we are just keeping some of the users on Citrix because it does allow a screen size percentage.