So many people do so many different things with nest boxes. I think we worry about them a lot more than the chickens. The main thing is to make them convenient for you.
The rule of thumb is to have one nest box for every four laying hens. They will mostly lay in just one or two anyway, but having enough is not a bad thing. I like a minimum of two anyway, even if for just 3 or 4 hens. If a hen goes broody, you may appreciate the extra nest box.
A few things I consider.
If they are too low or on the floor, you take away coop space for the chickens. This is not an issue unless you are tight on coop space. Another reason bigger is better.
Do you have a bad back? How much do you want to bend to get the eggs.
If they are not high enough for the chickens to get under them, that can become a very safe place for Mama Mouse to raise a family. When deciding how much space you need for chickens to get under there, don't forget to include the bedding. They tend to scratch loose bedding under the nest boxes so give it a little more space than you think you really need if you raise them.
If the nest boxes are real low, the chickens may hang out in them even when not laying. Also, they do scratch a lot. If the nest boxes are real low, they can scratch filth and trash in there.
Hens may decide that under the nest box and on top of the bedding is a good place for a nest. You need enough room to check under there and get eggs if necessary.
I know I've made it sound real bad to put them on the floor, but many people do and really like them there.
They do need to be lower than the roosts. Whe sizing them, I consider the minimum for a full sized chicken to be a 12" x 12". I made mine 16" x 16" and don't regret it. Sometimes hens like to crowd into a nest with two or even three laying at the same time. Sometimes one hen will not allow another hen to get in the nest with her, no matter how big the nest.
The way I determine height: I like a 5" to 6" lip on the bottom of the nest box to hold the bedding and eggs in. Many hens will scratch and scrape in the nest to get it just right before she lays her egg. If there is not enough of a lip, she can scratch out the bedding and any eggs that are already laid. Then I make the opening for her to get in about 6" to 8" high. Then, you may need to provide a little more height depending on how you frame the nest box. A hen stands up when she lays the egg so don't try to get it too short, though that is usually not a problem. You'd be surprised at some of the places I've seen them lay.
Those cat litter buckets, 5 gallon buckets on a frame, or many other ideas work great. You are only limited by your imagination on what you can use. Good luck and have fun with it.
Just for fun, here are mine. They are too close to the floor and I did add more lip after this photo. Having made those mistakes is why I can comment on this so well.