All current image editors and painters use diameter value for brush size. Radius for brush size in PhotoLine was very unusual way to go. Also seems that radius delivered a lot of problems for developers in last few beta versions:
It was impossible to set brush diameter 3,5,7,9... and so on because slider steps where radius 2,3,4,5... which only gives diameter 4,6,8,10...
Same time Radius 1 gives brush diameter 1 which looked very unlogical.
Then developers add 0.5 steps. But sliders and input text fields became show different values.
Then they fixed sliders and input text fields problem, but then brush size number don't match to real brush size in pixels because It started from value 1 instead of 0.5.
As you can see, all those problems could be avoided if brush size used diameter from start. I suggested to use diameter instead of radius about 2 years ago