Simple math works out to about 32 Cubic Feet or about 1.2 Cubic Yards. (4ft wide X 4ft long X 2 ft deep = 32 cubic ft) You can put more in if you pile it higher, but you will need a tarp to cover the mulch or it will blow away as you drive down the road.
I have routinely hauled 2 yards of mulch in my trac. I put my tailgate down and flipout my extendhan five milese to go more ter then lay a tarp over it to fill in the gaps. I dont have to go more than 5 mile so it works out really well for me.