Just a little more info on my situation.
Every time I start the game I get the frozen start page. If I hit ( command + F ) I can get to the options page.
In the options page I go to the Graphics page and check full screen. Then I hit the ( X ) in the upper right conner, then I hit the save option. I usually have to do this several times ( 4 to 5 ) until it sticks. But it does end up staying in ( Full Screen ) mode with the resolution of ( 2048 x 1152 ) one level down from the max ( 2560 x 1440 ) in the case of my iMac.
What's odd to me is that apparently my iMac, OS X El Capitain and my NVIDIA card with t's drivers can play the game in full screen. It just takes me repeatedly asking it to do so. I'm no programer but if I can do it on my keyboard it seems code and be written or changed to get whatever isn't listening to pay attention and lock into Fullscreen without me having to do it manually.
Now I must stress this applies to my iMac, I know all machines can differ.
My two cents.