Never bothered to look to see if it could be made bigger than a 10. I mean for me there is no point for anything bigger than a 5X5 size brush almost lol.
Looking in the final sun data ini right now..........Results:
-Couldn't find anything about creating a new brush size. In fact the only thing I found, though I didn't do a very thorough look, was the standard brush size. I have that edited to that my brush size is always 1X1 instead of 2X2. So my guess is that if you wanted to start out with a 20X20 or anything over 10 for that matter you would have to enter it in for the standard brush size. Which IMO would just be a pain.
This is how mine currently looks....(found in the FSData.ini )
[StdBrushSize]
ClearTile=1
RoughTile=1
SandTile=1
GreenTile=1
PaveTile=1
WaterSet=1
By changing the RoughTile and setting it to 15 it did in fact give me a standard rough tile brush of 15X15.
That is the only way I saw how to do this.