Couple of questions... Will you be letting them out of this coop to free range? Is this coop going inside a larger fenced off area?
Being in a warm climate, ventilation is critical. I'm sure you know that. But space is also very important. A 10 ft x 10 ft open-air coop can be adequate for roosting at night and escape from bad weather, but won't be big enough to act as both coop & run for 20 Wyandotte's.
Just trying to ask questions and understand your needs or expectations before offering appropriate suggestions.