Chicken run - how big?

Well, I've had quite a bit of info on different kinds of chicken houses. My question now is about the run...

I'm thinking of starting with 6 - 10 chooks. The allotment will be their main free ranging area, and I think I read someplace that it would be wise to keep 2 beds free so I can rotate on a regular basis. How big a run would I need? Would a 12x6 space do for the rotation, or do I need a bigger space/less chooks? How often should the run be rotated? What do I grow on the *resting* bit so the chooks get  plants they love to eat and are good for them?

Ps: I'm planning my allotment layout, hence all the questions  

i heard you can rotate it as frequntly as you want ranging from 2-3 days, to a week. but be sure to move it every year to keep away bugs, rats, fleas, any critter. correct me if im wrong.

well I built my run and was told it was very big for my 4 chickens, it is 17ft X17 ft and when the chickens arrived it was grass and weeds, within about a fortnight there wasnt a blade of grass in sight or a weed either, they stripped it bare apart from the azalea bushes.

so
now they have the permanent run to live in every day and I give them a sort or play pen made of old fireguards (very cheep and easy to move around) and this allows me to move them about the garden onto any bit of land that I want them to feed on, Ive got them under the apple tree just now eating any bugs that come from the fallen apples this time of year.
It works really well so I would suggest you make something moveable but not nickable if its on your allotment.