How can I restore default window size in KVM/QEMU

By pressing: CtrlAltu

Source: http://linux.die.net/man/1/qemu-kvm

In the future, you can also try to use man (in a terminal). This gives you the user manual. You can seek a word by pressing "/" (without the quotes) and then cycle through the references using "n":

man qemu
/restore <enter>
n
n

To quit, type "q".