Hi there! I can definitely solve that one for you. It looks like at some point the precise cursor option has been enabled, as well as the high visibility option we included so that people who use the precise cursor and want it a bit more visible can see it.
To use the normal cursor, just press "C" when in ArtRage, or you can go to your edit menu, select preferences and uncheck "Use Precise Cursor" in there. When you make your tools really small in Artrage, it always uses the precise cursor so you'd expect to see the crosshair again at that point.
However, if you uncheck the high visibility cursor option in Edit, Preferences as well as making the above change, this will ensure that you just get the standard small crosshair when your tools become too small to reasonably display the circle.