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

 import XMonad.Layout.LayoutHints

Then edit your layoutHook by adding the layoutHints layout modifier to some layout:

 myLayouts = layoutHints (Tall 1 (3/100) (1/2))  ||| Full ||| etc..
 main = xmonad defaultConfig { layoutHook = myLayouts }

Or, to center the adapted window in its available area:

 myLayouts = layoutHintsWithPlacement (0.5, 0.5) (Tall 1 (3/100) (1/2))  
                   ||| Full ||| etc..

For more detailed instructions on editing the layoutHook see:

XMonad.Doc.Extending