Welcome to BYC.
The Usual Guidelines
For each adult, standard-sized hen you need:
4 square feet in the coop,
10 square feet in the run,
1 linear foot of roost,
1/4 of a nest box,
And 1 square foot of permanent, 24/7/365 ventilation, preferably located over the birds' heads when they're sitting on the roost.
So if you keep all 13 chickens you will need:
52 square feet in the coop,
130 square feet in the run,
13 linear feet of roost,
4-5 nest boxes,
and at least 13 square feet of ventilation.
However, these are *guidelines*, not hard and fast rules. I suggest reading this excellent article:
https://www.backyardchickens.com/articles/how-much-room-do-chickens-need.66180/
Since you live in Texas, ventilation and shade are going to be the #1 factor for keeping your chickens healthy and happy. If your chickens have constant access to a big, shady run they'll be more willing to share a little less room in the coop than if they have to retreat to the coop to get out of the sun all the time. (I currently have 5 hens living happily in a coop built for 4 but have docile, laid-back breeds and massively over-sized run).
This
Texas coop was my main inspiration for my new, Open Air style coop (
currently being built).