To alter your sun ray's resolution, press Shift-Props and wait a few seconds. The Props key is on the very left edge of your Sun keyboard. If you don't have a sun keyboard, you can also run /opt/SUNWut/bin/utsettings to force the application to appear. Once it pops up, choose the display category and set your desired resolution. As always, be careful not to set a resolution that your monitor cannot handle.
You may also need to adjust your desktop size. It does not automatically change to match the new resolution, so it may not fill the screen or may extend beyond the viewable area of the screen.
To adjust your desktop size, you need to use utxconfig. For example:
/opt/SUNWut/bin/utxconfig -r 1280x1024
...assuming your desired size is 1280x1024. Once you've set the desired desktop size, you will need to log out and back in. See the man page for utxconfig or run /opt/SUNWut/bin/utxconfig -help for more details.