This similar thread may help
viewtopic.php?f=16&t=19342&p=270322#p270322
On the Iconbar in RISC OS there should be a picture of a screen, just to the left of the RISC OS 4 Icon. Click (Select) on it.
The window that comes up should show the name of your Monitor Definition File (MDF)across the top.
If you click (Select) on the drop down just to the right of the Resolution field, it will show you all the resolutions available in your MDF.
There may already be a similar one to what you are after.
If you want to add another and adjust / play with it in some way then :
Open up !Boot.Resources.Configure.Monitors
Find the file that matches the name you found earlier (there maybe a subfolder under Monitors)
Make a copy of that file to work on.
Using !edit or any other RISC OS editor of your choice, copy in any of the monitor definitions which may be of use.
It is worth giving it a new name where it says monitor_title in the MDF, so you know which one you are working off.
You can test it by dragging it to the 'Screen' Icon on the IconBar. Then if it all goes pearshaped a reboot wlll bring your working file back into play.
If you need to adjust any of the parameters load up MakeModes (see thread above) and play around with that until you get a mode you are happy with.
If you are 100% happy with it all. Then you can copy it into the boot location mentioned earlier.
Then use the Boot Monitor Config to make this your default MDF. Make sure you are 100% happy with it though, as if you mess it up, you may end up with no screen display at all when you start your machine.