I made mine last weekend...
I made mine 15x15 in the rear and 12x15 for the front both 7 boards high. I made a "limit" sticking up on one of the front just as one more little thing to help just in case it rolls.....doubt it will and that piece aint gonna hold much back but I figured what the hell..
I'm going to be doing a 335-35-20 rear and 245-35-19 front.
I had a bunch of small scraps so I put one of them into the gaps...same thing...what the hell...alittle added strength
I'm gonna keep jackstands under there just in case......