Here is the proper way to change your screen size.
Step 1: Load Risc OS to the desktop
Step 2: Double Click on the !Boot Folder to open the Configuration window.
Step 3 Double Click on the Screen Icon in the Configuration window to open the screen window.
Step 4: Select the monitor type generic by clicking on the button to the right and select generic from the drop down list.
Step 5: select the number of colours by clicking on the right from the drop down list.
Step 6: select the resolution by clicking on the right button and select the resolution to suit you monitor from the drop down list. no need to change frame rate.
Step 7: Click the try button to check if it works it will ask you click continue to test the settings, switch modes then ask if you would like to keep the new settings or discard them.
Step 8: most important part click the set, this will save the mode string into !Boot/Predesk/Configure folder, to use every time you start Risc OS