Nest boxes - How big?

[IcarusSomnio]

How big should I make my boxes? As far as adults go, I have 4 silkies and 1 Sebright, no roos. I got 5 Welsummer chicks, 3 Sebright chicks, and 4 Rouen ducklings today. I'm crossing my fingers and toes for hens

I currently have a portable coop but the 'laying' area is more of a perch than anything. I want to use it for my pigeons once I get a bigger one built for the chickens. Hoping to do that pretty soon, as I'm picking up a rooster this Friday

 

[ShaggysGirl]

mine are about 12 x 12. In my small coop I use an old TV stand type thing lol
This one I got lucky with a friend gave it to me