I don't use rdesktop from the command line anymore, but I used to use it over ssh that way and I still have following script (called from a desktop shortcut). Just tried and it launches rdesktop correctly in fullscreen no problem:
i have rdesktop version 1.7-maemo3 installed. Make sure you are on that version (I seem to remember full-screen issues & problems passing keyboard input to the app in previous rdesktop-cli versions).

If you're on the latest version already, maybe try reordering your parameters in case that makes a difference? or maybe try launching xterm from a script like above in case that's significant? I can't remember now why I started my ssh in background and then ran the rdesktop as a separate background process (rather than just putting them as sequential calls like in your method above) but I guess there must have been some timing issues in getting the tunnel up over gprs - I doubt that has any relevance for you. same goes for the fact that I am calling via tsocks (I needed that to have rdesktop use my socks proxy in order to remotely connect over 3G/public wifi to a 3rd party system via my own desktop in order to fool 3rd party firewall into thinking the traffic was coming form my home IP). I now have openvpn set up so I don't need any of the ssh/tsocks, I just connect from n900 to my home vpn and then run the rdesktop gui on n900 from within my own LAN in order to connect to the 3rd party from my WAN static IP (for which the 3rd party has a firewall exception).