How to change size of split screen emacs windows?

With the mouse, you can drag the window sizes around. Click anywhere on the mode line that is not otherwise 'active' (the buffer name is safe, or any unused area to the right hand side), and you can drag up or down. Side-to-side dragging requires a very precise click on the spot where the two mode lines join. C-x - (shrink-window-if-larger-than-buffer) will shrink a window to fit its content. C-x

[InoHiro]

"C-x +"(balance-windows)