This thread shows many ideas on nesting boxes. I think you will find there are a tremendous number of different ideas on nesting boxes. I do recommend you use something with a top to it to reduce potential problems of them roosting over them.
https://www.backyardchickens.com/forum/viewtopic.php?id=173767
Many people put theirs on the floor. Some put them fairly high so they don't have to bend to gather eggs. The usual recommendation is to put them 18 to 24 inches above the floor so they don't get as dirty. Mine are about 12"up for what that is worth.
For 6 hens, 2 is enough. I'd recommend a minimum size of 12" x 12". Mine are 16" x 16" x 12" high. Often the hens like to lay together and I like to give them some extra room.
The roost should be clearly higher than the nesting boxes, usually 12" higher is enough. Chickens tend to roost on the highest place they can get. You don't want them roosting in the nesting boxes because they poop a lot when they sleep and you don't want to have poopy nests. Obviously, that gives you dirty eggs.
In colder climates, many people use a 2x4 with the flat side up. The theory is that they sleep on their feet, thus keeping them warm. In warmer climates, the 2x4 with the thin side up is fine. The corners should be rounded as it is more comfortable for them.
I used tree branches. Mine tend to roost on the thicker parts of the branches, but that may be because of the position relative to the window, any nighttime breezes, or something else. I'm not sure. If you use tree branches, you should remove the bark as the bark gives roost mites a good place to hide. I'd suggest at least 1=1/2" diameter minimum size. And use wood, not plastic or metal. Plastic and metal are too slick plus conduct heat to well. They tend to get more frostbite on plastic or metal.
