I've seen just a few recommended sizes in different places, of course in places that I can't seem to find links for at the moment (links hopefully saved on the home computer). But, if I recall correctly, the tendancy is for the smallest effective area to be around 600 square inches when a mimimum recommended size is mentioned. This value should definitely go up as total system volume goes up, although I'm not aware of anyplace that says exactly how big a DSB should be for a system of x gallons.
Here's a link I found that goes into it a little:
DSB FAQ
There's also some info on WWM that talks about DSB size, but there's general recommendations there that some DSB is better than no DSB if someone wants a DSB.
I guess it's a matter of what each person wants in order for it to be effective. Any size DSB will support a certain number and type of sand bed critters, but I find it very hard to believe that a DSB that's a part of an average sump / refugium combination (where the refugium is at most 1/2 of the total sump and the sump is around 1/3 of the display tank volume) can eliminate nitrate issues in the average system with a moderate bio-load. It's my experience from working in a lab where others were testing nitrate to N2 gas conversion rate potentials in marine sediments that the process is slow. It's not something that's measured in a few hours, it's more on the order of a few days. The slower the process, the more surface area that is needed to ultimately control nitrate levels and keep them at 0 ppm. This is why I believe a DSB as a refugium chamber in a sump like the one pictured above has little benefits in terms of completely eliminating nitrates. I could certainly be wrong about this, but I haven't seen enough info to convince me that I am. I'd honestly love to be proven wrong -- it would help save me a ton a money on sand.
I run a 5g bucket RDSB on a 50g and it works great, but it's my understanding that each 5g RDSB can handle around 100 - 150g. I wouldn't think that one would be completely effective on a 200g, but if it's lightly stocked and not overfed, I suppose it's possible.