In KVM/QEMU machine window size by default matches guest OS resolution. But if I resize windows (accidently) how to restore default size?
By pressing: CtrlAltu
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".