Rocket mass heater in a tiny shed/ house conversion

We want to build a rocket mass heater in our tiny house.  Basically it's a shed that we are converting over.  I know the floor will have to be reinforced, but can a rocket mass stove be built small enough to heat under 400sqft without it running us out?

Sure can. Buy the Rocket Mass Heater Builders Guide.  http://www.ernieanderica.info/rocketstoves
My house is just as big and I have a full 8" system with 30ft of bench. You just won't have to burn it very long, get more heat in the mass than off the barrel so that you don't heat up the air to too high of temps.

I'm in a 12'x24' cabin with a loft. I have a 6" system with a 20' long mass bench - basically the burn area is at one end of the building and the mass goes the length. I burn maybe a cord and a half of wood per winter. On the coldest days & nights I use about 3 5 gallon buckets of wood, and I'm in New Hampshire. Coldest nights I burn for about three hours, and then an hour or two in the morning. It works pretty well, now that I've figured out its quirks and fixed a few things like making sure the chimney goes high enough.

When it is at the coldest outside I try to get it up to over 80 degrees F in here before going to sleep. Its a little hot, but not too bad, and when I wake up it is not too cold.

You will have to get used to the temp fluctuating but that's OK. You can also build the mass up a bit higher on the outside of the burn barrel to lessen the quick radiating heat during a burn, but you do have to be careful not to go so high that it dampens the heat riser/burn barrel temperature difference that makes it all work.

Awesome.  I was hoping we didn't have to go too long on the mass, since I'm worried about the weight.  That and it would change the entire layout we have planned.  You don't have the pipe doubled anywhere?

You can save weight by utilizing bells.  

Angela Pritchett wrote: You don't have the pipe doubled anywhere?

Mine is just a simple straight pipe. It goes out the side of the building and there's a T there, so it's an easy cleanout.

Most of the mass is concrete, but right around the burn area is cob.

What are bells? I've been watching and reading wherever I can and I know the term has come up but I don't remember what it is.

A bell is basically a hollow masonry box where the flue gases enter and exit near the bottom, and the hottest gases rise to the top, slowly give their heat to the mass, and cool and drop before exiting. The coolest gas in the bell is always what is going to the chimney. They are very efficient, and have next to no flow friction. If fed with a batch box, they can be reliably made smaller than a J-tube system, for heating a small space.
Real world applications: Bell with two benches.