How to change the landscape painting resolution?

I painted a landscape and it turned out to be a huge square with terrible resolution.

When I looked up how to solve this problem, it said to change the section size, but it is already at the maximum value and I can’t make it any larger.

How can I increase the paint resolution while keeping the current terrain? I only need high resolution for painting, not so high resolution for sculpting(I have performance concerns).

It took so long to make that I definitely don’t want to redo it. Someone please help me.