They can be on the floor if you want. I bought these recycle bins from lowes and they work perfect for bantams.
When stacked they use the lower one as much as the top one. At one point I had 3 sitting separately across the back wall with a broody hen in each. For standards they would have to be anchored because they are way too light. In the large coop with the standards I just used plastic storage containers on the ground. I had one with a door cut in the front and a lid on it and one that was open topped. Put in a couple inches of shavings and top with straw. They laid in both and the larger hens especially preferred the open topped one. I finally got a metal frame with 2 nests high (one completely on ground level) and 6 nests long that just needed some wood to finish it and now I only have guineas who are far more likely to use the storage containers or a dip dug in to the shavings in the corner of the coop. I'm debating how to make my nest box frame useful for rabbits.
Akane, your bantam looks like she could be my Pokey's twin sister!
Pokey is a Serama/Old English Game bantam cross breed.
I use plastic lidded totes as nestboxes, too. Love it! Easy to clean and easy to move around when managing broodies. You can just move the box with the broody and eggs inside without disturbing her.
p.s. As to the original question, I think the only real requirement is to make sure that your nestboxes are lower than your roosts so that your chickens will be less likely to try to roost in the nestboxes and foul them with droppings.