I use
rdesktop -g 1280x965 -a 16 -u user remote.host.id
where
user is the user name on the remote host and
remote.host.id is the hostname or IP address of the remote host.
1272x965 is a window size that fits decently on my 1280x1024 desktop while still leaving me enough room to see my jwm tray at the top and my wbar at the bottom.
16 is the color depth. I don't remember why I chose that value, but it seemed right at the time.