My Dakota with 6 1/2 foot bed holds 2 1/2 yards filled level. Bucket sizes vary, but you can probably calculate the cubic feet more or less, using lxwxh. 27 cu. ft. in a cubic yard. Your truck dealer or manual might tell you how many yards the bed holds.
Odds are that any stock 1/2 ton or less pickup filled with dirt to the volumetric capacity of the bed will be overloaded, weightwise. Even a full load of mulch pretty much flattens the springs on most of 'em, and dirt is more dense than mulch. Of course if you want get into that other thread that's running about weights and such, ignore all I have written in this paragraph.