Xmonad: Float and resize windows to its “natural size”

In xmonad she standard behaviour of windows is that they are tiled. One can change this, if one uses for example xprop | grep WM_CLASS and puts something like this:

myManageHook = composeAll
    [ className =? "MPlayer"        --> doFloat
    , className =? "Gimp"           --> doFloat]

Then the windows of those programs float and have a "natural size" (as they would appear e.g. in gnome).

Now my question is the following: Is it possible to do this "on the fly"?

I can click and drag it with the mod-button1 held down and then resize it manually. However, it should resize automatically to it's "natural size". What can I change in my xmonad.hs to achieve this?

Another possibility (which would be sufficient for me) would be to have some "starter script", lets call it xms: If I type xms foo the program foo is started floated and its windows have "natural size".