The tonnage is going to be based on two factors, the injection pressure and the cross sectional area at the parting plane. you will have to include your runners and sprue in the cross section calculations. This will give the bare minimum tonnage needed to hold the dies together, depending on how controlled the injection pressure is you'll most likely want to allow for a little variation and go with a bigger capacity. As for weight of the material just lookup the density and go from there, again be sure to include the volume of the runners and the sprue.