A box for dust baths -- how big?

[ScottyHOMEy]

I've lost three birds over the winter to what I thought was cold. They lost a lot of feathers to what I have thought was the one aggressive hen at the top of the order. Thinking mites as the alternative, I've been equally aggressive with dusting the birds and the coop.

With a smaller flock, and fair weather, the bird I suspected is showing signs of stress, as well.

Among my last ditch thoughts of saving any of them is a dust bath in the coop (they're averse to going out on the snow). My nest boxes are 16x14 and they only use two of them. If I need to eat up some floor space for something larger, I'll do it, but wondering if the third box will be large enough to serve the girls' purpose. It would be sharp sand with a generous mix of DE and permethrin in it.

 

[FireTigeris]

Sounds large enough if they are not tall birds, needs to be bid enough for a chicken sideways, flatways, wideways, longways - as they tend to really kick around in them.

If the bird can be laid down in the box w/o touching its big enough