Try: extras > options > player: fullscreen (on) & activate support for widescreen displays (on). That will stretch / scale the game in proportion to the target resolution of your machine.
If it's window mode you are wanting it to stretch in then I believe you need to add line to the config.ini file (of which I have no clue where it would be located on a mac. in windows it would be found somewhere in c:/users/username/appdata/local...
-- example for specific sized window with scalable content.
fullscreen = false
resizeable = true
resolution = 1280x720