Yes, you don't want to make them too small but your chickens need to feel cosy in them and so try not to make them too large either. The suggested ration is 3 or 4 hens per nest box but as many owners will say, it is very common for several chickens to favour one nest box whilst other boxes stay empty. Some people go further with their nest boxes and add curtain to add to the privacy. Anyway if you want to look at some of best advice from around the Internet regarding nest boxes then an article that I have recently written might be of some help. You can read it here http://www.allaboutchickens.info/better-chicken-coop-run/ and you want to specifically look at #17 (nest boxes). There is a quick navigation button to press that will take you straight to the section on nest boxes.
If the 12x12 seems small Id go 16x16. I belive you need 1 nest box per 4 or 5 hens.
ETA: 11 hens 3 nest boxes would be plenty. But 2 would probally work.
Nest size and height has more to do with what is more convenient for the human gathering the eggs than what the old hen prefers. If this was not so then how come you may find eggs laying every where and anywhere?Our boxes are 12x12 and we have some good sized girls and they seem to fit ok. we have 10 standards and 3 bantam hens, 4 are not laying yet. We have 6 boxes now which is overkill but we have overly spoiled chickens!
I like the 12x12 dimension. Use external mounted box to coop with a 9x9 opening from coop. I just built a three nest box, 3 ft x 1 foot with sides sloping 14 inches to 12 so the plywood top hinged to coop sheds water. The interior is simply two dividers so there is three nests. Works great for large fowl and I've had large, large fowl; you'd be surprised what they easily and prefer to get into to lay. For number of nests I think two is a minimum unless you've only 2 or 3 birds. Chickens like a choice so in giving 2 they then can all use the one. Though it does come in handy if one goes broody there is an open nest and with numbers of 9 birds the line to the nest gets long in morning so a few can't wait it out and use the other nest. Over 10 birds is time for 3 nests, over 15 birds or so 4 nests and so on. Start with two and have a nest for every five birds.Looking to build nest boxes soon.
I like the 12x12 dimension. Use external mounted box to coop with a 9x9 opening from coop. I just built a three nest box, 3 ft x 1 foot with sides sloping 14 inches to 12 so the plywood top hinged to coop sheds water. The interior is simply two dividers so there is three nests. Works great for large fowl and I've had large, large fowl; you'd be surprised what they easily and prefer to get into to lay. For number of nests I think two is a minimum unless you've only 2 or 3 birds. Chickens like a choice so in giving 2 they then can all use the one. Though it does come in handy if one goes broody there is an open nest and with numbers of 9 birds the line to the nest gets long in morning so a few can't wait it out and use the other nest. Over 10 birds is time for 3 nests, over 15 birds or so 4 nests and so on. Start with two and have a nest for every five birds.
Quick photo example of external mounted nests:
Bottom frame and two nailers for corners. sides sloped 14 to 12 inches. This gets screwed to coop.
2x2's on ends to screw the nest box to and middle ones to nail in a divider. Hole to coop was 10X30 making for 3 9x10 openings. Planned for 4 inches of bedding so bottom of hole was 6 inches up from coop floor.
Over sized top lid with two hinges mounted to coop.
