If you haven't already, try the Normal Brush cursor...
Thanks but - I already tried switching to Cursors from Normal to Full and back to Normal and this makes no difference - at normal or full.
You might have a corrupted preference file.
Reset your Photoshop Preferences.
OK thanks to the help in the retouching forum in DPReview - I have found out what I did.
MartinGus - Have you by any chance changed the cursor size in Mac OS? System Preferences> Accessibility> and then set it to "normal" (if changed). I had this problem with an earlier version of PS and after trying "everything" this solved it for me. Maybe a longshot since it´s not the same PS or MacOS version but maybe worth a try.
Me -- MartinGus - you are aces -- yes I had - so not a long shot at all -- and putting it back to normal fixes it -- BUT I now have the problem of not being able to find my mouse cursor -- Apple give you very few choices. Thanks to all I now know what I did to break this. This issue is now resolved.