You can use this module with the following in your ~/.xmonad/xmonad.hs:

 import XMonad.Layout.ResizableTile

Then edit your layoutHook by adding the ResizableTile layout:

 myLayouts =  ResizableTall 1 (3/100) (1/2) [] ||| etc..
 main = xmonad defaultConfig { layoutHook = myLayouts }

For more detailed instructions on editing the layoutHook see:

XMonad.Doc.Extending#Editing_the_layout_hook

You may also want to add the following key bindings:

 , ((modMask x,               xK_a), sendMessage MirrorShrink)
 , ((modMask x,               xK_z), sendMessage MirrorExpand)

For detailed instruction on editing the key binding see:

XMonad.Doc.Extending#Editing_key_bindings.