Larger diameters can work too, but there is a limit: if ducting is too large, then the system will have the potential of getting REALLY HOT. This means that the design will require different materials (with higher-heat-resistance) than the standard rocket-mass-heater stuff. There is a podcast by Paul Wheaton, Ernie and Erica Wisner, which talks about exactly this. The podcast is long, and they talk about a lot of stuff, but eventually they mention the maximum size rocket mass heater duct. You can get the podcast here: http://www.richsoil.com/permaculture/182-rocket-ma...ters-permaculture-podcast-019/
Looks really awesome though. A pile of fire bricks is a great way to get started!
Oh and another thing: I recently was testing my stove-core, which is made of bricks. I did a full mock-up, with a barrel and pipes and all. I applied some mud as temporary mortar to seal some of the main cracks and holes, but the darn thing just wouldn't draft correctly. It was getting 100% smoke-back. Not drawing any air in the proper direction. The next day I rebuilt the whole thing, again using that mud as temporary mortar, but this time I mortared between nearly every brick-- especially throughout the heat-riser. It produced a lot of steam (during the first hour or two), but otherwise it drew and drafted a lot better. So.. moral of the story is, when building semi-full-scale mock-up, don't underestimate the importance of mortaring all the cracks in the bricks.