For the most part, it shouldn't matter what sizes you use. The strength will be proportial to the sizes, and the sizes will be proportional to it's weight. So if you use smaller pieces, it will hold less, but weigh less. If you use larger pieces, it will hold more, but weigh more.
It is scored by (weight held)/(bridge weight). Highest number (strength proportion to weight) wins.
The only things were sizes could make a difference, it wether pieces are in tension/compression, or being bent. If pieces are in tension or compression, use square pieces. If pieces are being bent, use rectangular pieces. Sorry if I am over-explaining, or under-explaining. I don't know how much you know. If you want to know how to orient rectangular pieces, google "Plank v. Joist".