I was looking in the settings and saw that the max screen size (which is what I used for the first hour of play) had 60hz compared to smaller sizes that had 75hz. So I'm not sure what screen size I should use for the rest of the game. Could someone please help me?

60 Hz is referring to the refresh rate of your display. With 60 Hz it will draw 60 images per second. With 75 Hz it will draw 75 images per second.
If you have got a standard LCD, 60 Hz will probably the normal value of your display. I recommend to set the resolution to your LCD's native resolution and ignore the refresh rate.