The colour axis is an object of the type [[http://root.cern.ch/root/html526/TPaletteAxis.html|TPaletteAxis]]. 
Here the code i use to format it nicely:
<code python>
def ScaleZAxis(histname):
 	hist = ROOT.gDirectory.Get(histname)
 	ROOT.gPad.Update()
 	palette = hist.GetListOfFunctions().FindObject("palette")
 	palette.SetX1NDC(0.90)
 	palette.SetX2NDC(0.92)
 	palette.Draw()
 	ROOT.gPad.Update() </code>