In CS6 the only way to increase the size of the UI is to lower the screen resolution in your monitor settings.
InDesign CS6 does not support any high resolution displays. You have to upgrade to a Creative Cloud Subscription.
CS6 is not optimized for Retina displays, but it does work (and many users find the display quality acceptable). I run it here on a 30" Dell ultrasharp at 2560 x 1600 with no issues at all. On the other hand, the interface is a bit smaller than I like on my 17" laptop at 1920 x 1080, but I've learned to live with it.