The brush size in DxO PhotoLab (in Local Adjustments) is measured in pixels. Currently the size is limited to 500px which is too restrictive for 40MP+ images which are common nowadays, especially if Feather=0. Please consider to increase this limit at least 10 times. Selecting large objects with smooth and tiny brush is nearly impossible.
Update: I was completely wrong. I interpreted units as pixels (because the number looked equal to the to the number of pixels at the “Fit all” zoom level on my display). The comment by @Joanna below solves the issue.