I use a loop of 6 mm cord 5.5 m (18 ft) in circumference. Buy some larger amount of 6 mm cord, because the price for the pre-cut cord is a rip-off, and you'll end up going through this stuff for Prusiks and various other uses. You will of course need a little extra length in order to tie the bend.
There is no way to say what is the amount of cord needed for a 3-point anchor, because it depends on how your gear placements are situated. Mine is long enough maybe half the time. The other half of the time, I use slings to add extra length at one or more of the pieces of gear.
If you're extremely confident that your gear placements are bomber, then you can get away with a somewhat wider angle between the different strands, which lets you get by with slightly less cord. If some of them are not that great, you can pamper them by lessening the angle to decrease the mechanical advantage, but that requires more cord. If the pro really sucks, then you might want to consider an equalette setup, which will give you perfect equalization. Perfect equalization is never possible with the sort of anchor shown in the photo (but it's what I normally use because it's simple to set up).