I haven't had that problem. My trailer measrues 6-1/2 ft by 16 ft with 4 ft sidewalls. I load it level to the top loose and call it 2 cord. I know that it actually stacks out a little more than that, so my customers always get a little extra wood, and I can just scoop it out of a pile with my crawler loader and dump it in, or carry it from the woods in the bucket and dump it in. Now, if I could just get that trailer to unload itself!
I've often pondered the idea of using stackable wire baskets that would hold 1/2 a cord. Carry the basket into the woods on forks, stack it full, drop it on the trailer. Later you could fork it off (even store it in a barn and rotate the supply to always have seasoned wood.) Then, offer your customers the ability to rent the basket, drop off the wood, and they have a way to handle it easily (with the proper equipment). You could even build a hydraulic crane on the trailer to load/unload the baskets. If selling firewood was all I did, I could concieve of recovering the initial cost of the baskets, but as someone who cuts more for fun and has a full time job, it really isn't economical.