I just upgraded from QGIS 2.18 to 3 on my MacBook Pro (retina display, running High Sierra). Some of the cursors are enormous and I can't figure out how to change their size. For example, the Pan Map cursor is the normal size, but the Zoom In/Zoom Out cursors are about 2 or 3 times the normal size. Screen shots show what I mean.
I don't have this problem with my iMac, so I'm wondering what's wrong on my MacBook Pro. I don't think it's a retina screen issue, because it didn't have this issue with QGIS 2.18.