mmkay so, first you need to decide a length, 8, 10, 12, 14, 16 feet are your best bets, because those are lengths of lumber you can find at your local store. Decide how high you want the box, 2 ft is a good choice if you have a realitively flat yard space. now once you have the length and the highth, decide the width, if you wan't it to be any wider than 8" (basically if you want a box and not a rail), than you need to make a 2x4 frame and if wanted side the frame with 3/8" cdx plywood.
The frame should basically be 2x4 box shaped ribs. the ribs will be connected to the 2x4's that are the length of your box with each long 2x4 in the corner. like this:
if your box is smaller than 8" make a simple 2x4 rectangular frame like this:
now if you box is tall and skinny you might need stabalizers on the bottom to keep it from tiping if your short on snow, just screw some 2x4's to the bottom and you'll be fine.
after you have the box built your topsheet design can be a number of different options. first is lexan or acrylic, both are very expensive yet super durable, so if your packin an extra pound or two, or if you like 7 feet tall, this is a good decision. Second, and most creited, is HDPE (high desity polythylene), they use it to make puckboards in hocky rinks, it can most likely be special ordered by your local store, but ask for uv stabilized HDPE so it won't get brittle in the sun (if you don't live in england). both come in varying thicknesses and its up to you weather you want to glue it and screw it, or just glue it. use gorrila glue or contact cement. Coaping is an option, if you wan't the edges of the topsheet not to chip or shave here and there, ask your store if they special order HDPE rod, if not go to:
and get it thicker than your topsheet was. glue it to the sides of the topsheet and box itself.
finaly if your feeling creative paint your box, spray paint your box, stencils stickers, and what not. make a decent kicker and sesh the bejesus out of it untill the snow melts away, then bust out the turf.
my sugestion's for width is 1' and highth 2', length is whatever you can fit in your yard, and whatever you can carry around, have fun with it, Byeahhhhhhhhh!