Windows size for Mac Retina?

Hi -

First, I’m completely new to Mac development so please forgive my ignorance.

I’m having a hard time figuring out what size I should make the window for my game to take advantage of Retina display? I’ve tried 1280x720 and it takes up pretty much the full screen, but it decides to use the 1024x768 resolution assets (which makes sense because 720 < 768). If I go bigger than 1280x720, the window stretches past the edges of my monitor.

So, my question is, what’s the “correct” window size and design resolution to take advantage of the Mac retina display?

Thanks