Is it possible to set the viewport render maximum resolution when using Cycles? When working on a retina MacBook Pro screen, the cycles viewport renders feel slow compared to a non-retina screen because it's rendering at full retina resolution. While this is sometimes useful, most of the time I'm looking for fast feedback when lighting, so a coarser but faster resolution would be better. Is there a setting or a way to limit the viewport renders to bigger pixels without running the entire application in low resolution?
There isn't really a way to do that yet. You can play with the Start Resolution under the render properties panel to force Cycles to start rendering at a lower resolution, however, it will still eventually progress to a full, retina-resolution render.

Although one could argue that because of the high resolution, it takes less sample iterations to get to a visually 'smooth' image (even though each sample iteration does take longer), so you can probably try lowering the Preview Samples count.