Shift + Space Doesn’t Work on Mac
Shift + Space is the default key mapping for maximizing the window. However, it doesn’t always work on Mac. One way to fix that is to go into Preferences -> Keys -> Window / Maximizing View (the last one) and uncheck the Shift modifier flag. Then simply hitting Space will maximize the window. Of course, you could change the keys as you like.
If you don’t want to do that, you can pinch to zoom in and zoom out.
Bonus Mac Unity Tips
Two finger scroll to zoom in and out
Hold Alt while holding down the Left Mouse Button to rotate around a point
Hold Alt+Cmd to move around the viewport