AS has been mentioned, the Hogue Handall is the cheapest way to make the grips 'bigger'.
Also, the rubber Hogue replacement grips are bigger than the factory grips, as are the many nice wood grips Hogue still sells for the 5906.
I bought a very nice set of the wood grips for my 5906, but found that I didn't like the larger size: I have fairly small hands, so I sold them.