What size should the door be?

[the petgardener]

Hello to BYC.
I am in the process of "re-fitting" our wooden play set into a chicken coop.
During the hottest time of the year - but luckily there is plenty of shade in this 98 degree weather.
My question is this - I have Black Astralorp, Dominique and Golden Buff chickens (almost 4 weeks old now and getting HUGE) and I want to make sure I don't make the door too small (or too big)
The auto opening doors are not in my budget (using all reclaimed wood)
I was thinking 18" tall x 12"wide?
The door will be a slide - pulled up by a cable on the outside of the pen.
Any help would be greatly appreciated

 

[Hokum Coco]

My door allows only one bird out at a time. It is only 9"x12". I have standard size chickens Delaware & Golden Comet. I wish I had put my door 12 inches up from the floor rather than 6".

 

[MoonShadows]

18" x 12" is more than sufficient.