How can I resize the game?

[Gibstick]

See the commandline docs for information on how to launch AC with a set screen resolution. A simple way to add switches is:
1. right click on your AssaultCube shortcut
2. add the switches to the very end of the Target box outside all quotes.

For example, you might want

"C:\Program Files\AssaultCube_v1.1.0.4\assaultcube.bat" -w800 -h600