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

    import XMonad.Actions.FloatKeys

Then add appropriate key bindings, for example:

  , ((modMask x,               xK_d     ), withFocused (keysResizeWindow (-10,-10) (1,1)))
  , ((modMask x,               xK_s     ), withFocused (keysResizeWindow (10,10) (1,1)))
  , ((modMask x .|. shiftMask, xK_d     ), withFocused (keysAbsResizeWindow (-10,-10) (1024,752)))
  , ((modMask x .|. shiftMask, xK_s     ), withFocused (keysAbsResizeWindow (10,10) (1024,752)))
  , ((modMask x,               xK_a     ), withFocused (keysMoveWindowTo (512,384) (1%2,1%2)))

For detailed instructions on editing your key bindings, see XMonad.Doc.Extending#Editing_key_bindings.

keysMoveWindowTo (x, y) (gx, gy) moves the window relative point (gx, gy) to the point (x,y), where (gx,gy) gives a position relative to the window border, i.e. gx = 0 is the left border, gx = 1 is the right border, gy = 0 is the top border, and gy = 1 the bottom border.

For example, on a 1024x768 screen:

 keysMoveWindowTo (512,384) (1%2, 1%2) -- center the window on screen
 keysMoveWindowTo (1024,0) (1, 0)      -- put window in the top right corner

keysResizeWindow (dx, dy) (gx, gy) changes the width by dx and the height by dy, leaving the window-relative point (gx, gy) fixed.

For example:

 keysResizeWindow (10, 0) (0, 0)      -- make the window 10 pixels larger to the right
 keysResizeWindow (10, 0) (0, 1%2)    -- does the same, unless sizeHints are applied
 keysResizeWindow (10, 10) (1%2, 1%2) -- add 5 pixels on each side
 keysResizeWindow (-10, -10) (0, 1)   -- shrink the window in direction of the bottom-left corner

keysAbsResizeWindow (dx, dy) (ax, ay) changes the width by dx and the height by dy, leaving the screen absolute point (ax, ay) fixed.

For example:

 keysAbsResizeWindow (10, 10) (0, 0)   -- enlarge the window; if it is not in the top-left corner it will also be moved down and to the right.