heatsink/exhaust limits

I saw a video a while back where a guy on a bike towed a rocket mass heater in kit form to a farmers market or local fair. Once there he used the trailer box filled with rocks to run the exhaust pipe through.

Last night, for no apparent reason I had the idea of building an 8' x 10' raised bed, running the venting for a rocket stove through it and then filing it with rocks and sand and the covering it with decking upon which to pitch a wall tent. Think glamping with in floor heating.

So my question is whether there is a limit or formula that will help me figure out if my venting is too long or too wide or what the perfect combination on flue diameter and length would be?

Andy Hawkins : The Video you originally saw was at richsoil.com, which is a sister site to Permies.com ,go back to the home page and click on Rocket Stoves, then on that
page scroll down to and watch that video again, Note the number of Athletic people with trailers (that they were used to using regularly) it took to transport that portable
R.M.H.. And then on top of that you want to transport decking and a wall tent ?

I have a favorite camping site which allows me to use an old hollow tree well banked with cob as a chimney, ( talk about your wood stove ) And an old beaver dam as my
own private wood pile, A reflective blanket and a hammock complete my kit! Less is More ! Big Al!

I'm not looking to transport anything that was just the original video that inspired the idea/question.

The 8'x10' heatsink will be a permanent fixture as will the walltent