According to Storey's Guide to Raising Chickens, Hens like to lay eggs in dark, out of the way places. Discourage non-laying activity by placing nests on the darkest wall of your coop or shielding the nests from light. Here are their recommended sizes:
Leghorn size layers: 12"w x 14"h x 12"d
Heavier breeds: 14"w x 14"h x 12"d
Bantams: 10"w x 12"h x 10"d
They also say that a rail just below the entrance to the nest gives hens a place to land before entering. For most chickens the rail should be no closer than 8" from the edge of the nest to discourage roosting and pooping in the nest.
You can furnish a community nest, but any next large enough to accommodate more than one hen at a time invariably leads to broken eggs.