first, you should be using either the new Unity UI system ( ugui) or a good, up to date and battle tested UI alternative, such as nGui.
I'll concentrate on uGui:
uGui has a canvas scaler component which is where you'll be setting how your canvas behave depending on screensize and dpi. For quick setup, I usually pick "Scale with Screensize" and set the MAtch to be width and height at 0.5. don't forget to properly design everything using the reference resolution you pick.
fitting all resolutions with one setup is still something that I believe is not available, you'll have to change your scaling depending on retina in some cases, simply by checking which device you are running on, you can do that with PlayMaker yes,