Problem with RealVnc on Raspberry pi

[mcgarrydware]

I have setup a raspberry pi with a Hat to receive AIS signals and also have connected a GPS receiver via USB. All that works fine and I can connect to it with my Android tablet using Navionics app. I can also SSH into it from my raspberry 400 that I use as the main computer on the boat. RealVnc used to work fine until I decided that I didn't like the resolution it was set at. As soon as I tried to change it I messed it up. Now all I get is the top left hand corner of what should be the desktop of my raspberry pi. I have tried endlessly to change it back but to no avail. Any help would be much appreciated.

Mick