Change VM resolution to specify resolution, for example, 1366×768 [(https://superuser.com/questions/443445/how-can-i-get-virtualbox-to-run-at-1366x768)]

VBoxManage controlvm <name-of-vm> setvideomodehint 1366 768 32

Resize VM hard disk [(https://askubuntu.com/questions/88647/how-do-i-increase-the-hard-disk-size-of-the-virtual-machine)]

VBoxManage modifyhd /path/to/YOUR_HARD_DISK.vdi --resize SIZE_IN_MB

Usage:

rdesktop -g 1360x760 -P -z -K -x l -r clipboard:PRIMARYCLIPBOARD 127.0.0.1