Are you using Windows? If so you can use a registry tweak to change the window size.
Close Spiral Knights (if it's running).
Click on the 'Start' menu and then click on 'Run'.
Type in "regedit" and hit 'OK': this will bring up the Registry Editor. Warning: Please do not change anything else in the registry since it could cause severe problems.
Browse through the registry tree and expand to 'HKEY_CURRENT_USER\Software\JavaSoft\Prefs\projectx'
Look for the value "display_mode" and double click it.
The first two numbers represent window width and height. Set these values to what is desired. NOTE: Only values seen in the in-game drop box will work. Others will be ignored.
Close the Registry Editor and start Spiral Knights.
Are you using Windows? If so you can use a registry tweak to change the window size.
(this was adapted from section 5 of the Technical Help Sticky
For what it's worth, you should also try to seek out a driver update for your graphics card.