I’m running Aquamacs on my Macbook. One day I started it and it had no resize handle in the lower-right corner. There was no way to resize the window. Maybe using “(set-frame-size (selected-frame) )" in Emacs would have done it.

It turns out that the “emacs starter kit” killed it.

The solution is easy. Do NOT disable the menu-bar. I commented out the following line

;;(if (fboundp 'menu-bar-mode) (menu-bar-mode -1)) 

Do NOT disable menu-bar-mode under OS X, because it takes the resize handle with it.