How to make double click on titlebar maximize a window to fill up screen? I hide my dock. How do I acheive this: double click on titlebar to maximize a window to fill up screen?

Please pay attention to two red rectangles (not fill up the screen though I double click on titlebar) as shown below a screenshot of the entire screen. 



This post How to maximise a window to fill up screen in Mavericks? proposes to use SizeUp (PS: it is not free?) by assigning a new shortcut. Is it still possible to use double click?
BetterSnapTool has the option that you're requesting. (The drop-down has many options including maximize.) You can also change the behavior of the green zoom button to do this as well.