Assuming you mean the bolt you tighten that clamps it all down on the handlebar, and the Shifter/Brake Lever combination.
Then M8x16mm and there should be a thin washer to make it a better and smoother action.
Quick search for ST-M950 and it's diagrams shows this.
Kindly SJS has a nice picture of the official one, though get a domed head might be easier. A Cap head (large cylindrical, might mean the bolt gets in the way of your finger. I don't know. http://www.sjscycles.co.uk/shimano-st-m ... prod21342/