Hi, it is not possible to change the size of the brush during use in Lightroom on mac.

There are two ways of changing the brush size during use. One is using the "[" and "]" keys, which decreases/increase the brush size. The other is using the scroll wheel on a mouse (if you have one). Both are working fine on both my Mac and Win10 systems. Have you tried both methods and are therefore saying they're not working? If so, can you provide some details of software version, Lightroom version, and what happens when you try to change the size? Any error messages?