If you think the maximum brush is too small, my guess is...
... you love the checker block effect
... have not discovered yet that you can select a large area and just fill it
To fill a large / huge area, use the "Select area" option found in the bottom left of the Paint tab, select and area and click on the "Fill area" button found to the right of that button previous.
But unless you are painting grain fields or something similar, it would not use it at all and paint your landscape with the smaller most possible brush while keeping one of the rotate buttons pressed.