It depends entirely on the make and model of the cistern you're using.
The flush volume is determined by the height the water gets to during the fill, and level the water falls to during the flush.
Different cisterns have different ways you set the fill and the flush. These are normally detailed in the instructions that come with the cistern, e.g. http://www.grohe.co.uk/lib/1/tpi/1241622.pdf
In the absence of instructions, if you raise the position of the float (so it sits higher in the cistern) without moving the valve, you'll generally increase the fill.
Chumpy, September 2009
