Photoshop brush size

Yeah, in addition to @PECourtejoie 's workaround using Alt+Scrubby Slider to do the fine adjustment to the exact number (attaching illustration), you can also click in the field and use the arrow keys to fine tune the size. 

Hi, a good way to get fine increments is to use Scrubby Sliders functionnality.

Hover the "Size" text, and click-drag to the left and right.

If you hold ALT / Option, you will get finer increments. (Hold Shift for larger increments)

You can also hover the numerical field, hold CTRL (PC) or CMD (MAC) then click and drag (adding ALT for fine increments)

I can definitely see what your referring to when using the sliders.

My testing was done using both a mouse and tablet.

Here with photoshop 2022 (23.3.1) on macOS Monterey the brush size adjustment, when right clicking on an document or using the Brushes panel cannot be adjusted in single increments of brush size, in other words, sizes are skipped as your example going from 7 to 9. On the other hand, if one does the adjustment from the Brush Settings panel, brush size increments are not skipped.

In photoshop cs6, it seems that all the brush slider adjustments do not skip increments.

Use bracket keys [ ] to enlarge or reduce the size of the brush cursor.