A really annoying aspect of rdesktop is that it defaults to a colour depth of 8 bits and a geometry of 800x600, and hence it's necessary to specify options on the command line if you don't like these defaults.
The following patch allows the user to have a config file (located in ${HOME}/.rdesktop/config, to automatically read in some defaults.
Example config file:
colordepth 16 geometry 1024x768 fullscreen 0
The patch can be found here: rdesktop-1.4.1-cfgfile-0.1.patch