Resolution Issue on iMac (4906 x 2304)

I have been a long time player of DST and recently purchase a 21.5" iMac. I changed the resolution settings to 4096 x 2304 (which is the resolution my mac runs at) and am running into issues. I can play but for some reason my mouse is very very small. This is causing my mouse speed to be very slow despite that I use a somewhat fast mouse speed. Is there any way to fix this? The 4096 x 2304 also has part of the menu cut off around the edges but during gameplay this is not an issue. With the mouse being so small and the speed affected, I can't really play the game normally. I tried other resolutions but it causes major issues like lag and distortion. Can someone please fix this. I included a screen shot of my specifications. 

Thank you!