For the ventilation I am using wire mesh
Not sure, but I think you meant hardware cloth? Half inch openings will stop most things, and it also comes in quarter inch which will keep out snakes.
You will be very glad it's a walk-in coop! Your back will thank you for thinking ahead. If you can run electricity out there, you will give yourself some options with light or heated water, though you may not need the latter where you are.
Using the dimensions you have makes efficient use of common building supplies, which are often 4x8.
At 8x12, you have room for lots of chickens (or other birds). Enough space and ventilation are two big things to have in the coop. It sounds like you are on the right track.