4x2 should do ok just for storage unless your puting up anything really heavy but i would have put them in at 400 centres. flooring panels all come in sizes that fit in with either 600 or 400 centres so will work in if marked out properly but id also make sure the end joints land on a timber joist , id also put a 4x2 runner bolted into the wall under the joists you have fitted on hangers plus the joists would have been better with a bit more overhang on the metal roof truss, they look awfully near the edge.