The guy's at Umwelt und Projektwerkstatt Freiburg e. V. (Germany) are developing a three wheel trailer for heavy loads. It's named Carla Cargo and it's Open Source Hardware.

Their documentation is just at the beginning (they spend their time more with building and trying out new stuff, than with documenting) and until now only in German.

The dimensions can be seen at Werkstatt Lastenrad.