Help with joist size please.

Hi,
I'm planning on building a large garage and need help choosing the right size roof joists.
The building will be 4.6 meters wide and approx 5 meters deep.
The joists will span the width and the roofing material will be galvanized corrugated steel so nothing to heavy.
As with most things I need to do this as cheap as possible so any suggestions on type of wood and good suppliers in the Bridgend area of Wales would be great.