One Place Studies can be any size, from a street or hamlet up to a town or even wider area (see http://www.wirksworth.org.uk/
for one of the larger ones). Ideally it will be small enough that you can make progress without getting bogged down, but still large enough you have a lot of variety in the families of that place. I would suggest looking at the census for each year - around 250 to 500 people is ideal. If you go bigger it can certainly be done (Wing has around 1500 to 1800 people each census), it will just take you longer to transcribe key documents like the census and the parish registers.