To do this, follow these instructions:

1. Click File | Defaults.
2. In the Settings field, expand Miscellaneous.
3. Select PlotSize.
4. Enter the new maximum plot size in the Setting value field. 
   
5. Select a Setting Persistence value (use Always reset to always keep this plot size).
6. Click OK.
7. Click Yes to save the changes to the surfer.set file.
8. Restart Surfer. 

Updated October, 2020