I seem to recall that both on Mac and Windows there could be OS-side reasons for cursors displaying incorrectly.
Do you have any OS plugins installed that might be involved?
What have you done for trouble-shooting so far?
Restoring Preferences after making sure all customized presets like Actions, Patterns, Brushes etc. have been saved might be a good starting point:
If you're using a drawing tablet like a Wacom, this will be determined y how hard you press.
If not, this could be the type of brush.
Try a standard brush with 100% hardness. Click and Shift+ click in a different location to test.
you should get a straight line between the two points
I have tried resetting the brush tool. I also have created a new photoshop preferences file by moving old one out and restarting PS. I do use a Wacom tablet but checked the opacity and flow settings and don't think this is the problem. When using a mouse with the tablet removed the same problem occurs. I have a mac and both the OS 10.12.6 and photoshop cc 2017 are up to date.
What are your System Preferences > Accessibility > Display settings?
Thanks a lot, my cursor size was set to between normal and large, put it back to normal and Volila!
Thank you, phew.