It looks to me like the button sizes have returned to the size that they were in 6.17 (and earlier) now that 6.21 has been released.
In the options dialog, I have the button size set for small, and it's almost identical to what I had in 6.17 (visually on my system the difference is an extra one pixel in the width, and no difference in the height).

If Khaled did bring back the option to control the height & width of the buttons (ie: allowing you to set the number of pixels) then I was unable to locate the setting.