Well, larger sized pieces are going to allow for faster flow through the media while smaller, of course, will slow down the flow, but allow for better mechanical filtration. Seachem's site says Matrix is about 10mm in diameter. That would be an average since some pieces are a bit smaller and some a little larger. ADA's Bio Rio, which is a volcanic rock, is quite a bit smaller than Matrix, probably less than half the size of Matrix. I just mentioned those two for comparison sake. The size you've mentioned would work well also, IMO.