A 4x8ft sheet of plywood will make 4 - 2x4ft shelves, and if you'll get treated 2x4s, they're stronger than plain lumber, and can be ripped in two for less bulky legs and cross supports, and to make your material go farther. It's really simple to make some very large and sturdy shelves, especially if you use decking screws(works best to drill the first part so it will pull in tight to the next one). If you want to be really cool with it, use a wood rasp(it looks like a file with round ends, and has several different size tooth patterns for coarse and smooth rasping) to round and smooth all the cut edges, and then run sandpaper over the tops, sides, and edges