Q: How large should my nest boxes be, and how high should they be off the ground?
A: Some people put their nests directly on the ground, although this is not ideal since other hens may stick their heads in the nest to see what the laying hen is doing inside. (Generally the girls don't like to be disturbed while they are laying.
Other people raise nests a bit; it depends on the style of your boxes and your preference. They should always be lower the their roosts, however, or you will have them trying to roost in the nests! If they begin sleeping in their nests, the nests will get dirty, and by consequence you will have dirty eggs.
As to size of the nests, it is not an exact science. Generally, it should be cozy without being tight. Because chickens come in different sizes themselves, you have some leeway. There are even different sizes of bantams: seramas, for example, are so small, some look to me to be about the size of a large robin!
You can also use "found" items as nests to save money. Many people use old drawers, and covered kitty litter boxes are a favorite, too. Some people fashion nests out of plastic storage tubs. A tangerine crate made a great "broody" nest for my bantam Silkie hen to hatch her eggs, but when she's in the coop with the "big" girls, she lays in the big nests with the large fowl.