Jean-Louis Arcand, Enrico Berkes, Ugo Panizza summarize the conclusion of their research into the size of the financial system in relation to the economy in Too much finance? at voxeu.org.
"Over the last three decades the US financial sector has grown six times faster than nominal GDP. This column argues that there comes a point when the financial sector has a negative effect on growth – that is, when credit to the private sector exceeds 110% of GDP. It shows that, of the advanced countries currently suffering in the fallout of the global crisis were all above this threshold."
Short and to the point. Useful read.