If you make anything, make it 10 foot wide. I made two of them, 1 - 10 by 16 made out of 3 X 12 inch bridge plank on a 8 ton gear and the other 10 by 24 made out of 4 X 16 inch bridge plank on a 12 ton gear. The 10 foot width is really nice to set 2 bale side by side on the rack rather than having a foot sticking off both sides of the rack.

Stretch the gear out so it is a 3 or 4 foot shorter than the rack. this way, if you put a bale on the front or back first, the rack will not want to tip up. Sometimes you need to get a new pipe for the reach to do this.