You can adjust the res by pressing CTRL ALT and the + or - key on the keypad.
To make the change permanent (you know how big you want it) you need to edit the screens section in your XF86Config file. The order needs to be as you want it from preferred size to not-preferred, it's most likely smallest to biggest currently. So let's say you want 1024x768 to be the default, you will need to edit the section to have:
1024x768, 800x600, 1280x1024
Rather than whatever is there (substituting all the options I've shown with the ones you desire). This will only need to be done with the Default Depth that is displayed (24 or16 probably). Each depth has it's own section, you only need to edit the section corresponding to your Default Depth.