when you get into the Workbench, go to Prefs > Screenmode chose the right resolution and click the Save button.
Remember to click on the Save button, only if you are 100% sure that the Workbench is displayed correctly.
The size of the displayed Workbench window, can be changed with the Prefs> Overscan tool. There are other ways to resize a window in AmigaOS (f.e clicking on the little triangle at the bottom right of the window itself)
