Here the solutions...
1)If you want to open the screen at a specific resolution, for example 1280×1024 resolution, add the -g flag:
rdesktop -g 1366x 768 [machine]
2)if you like using
rdesktop -g 90% servername
which will use set the size of the remote desktop to 90% of the size of your local screen, which works quite well regardless of resolution.. :)