Depends on who you talk to. Some people recommend 2 square feet per chicken in the coop, some recommend 15, most are in between. You can get various responses for outside room too. I personally don't have a recommendation but think that there is no one number that can work for every chicken on the earth. If you follow the link in my signature you can see some of the things that I think are important.
If you can tell us your specifics, such as your goals for having chickens, your general location so we know climate, how much room you have to work with, how many you plan to have, how you plan to manage them, and such we may be able to make specific recommendations that actually fit your circumstances.
General rule of thumb for home keepers that is a great starting point is not less than 4 square feet in the coop and 10 square feet in the run... per female bird, for home keepers. Also at least 1 lay box for every 3-4 hens. Not less than 1 linear foot of roost per hen or 1.5 foot per rooster and an extra foot or so of maneuver room. This usually allows for time between cleaning
More space may be needed during times of harsh weather, to accommodate broody's, integrate new birds, or set up a hospital/quarantine/ broody breaker zone... And space needed can vary by personality...
I have kept 24 bantam roosters in only 32 square foot run with A frame coop above it for a period of time. It wasn't ideal, and I won't do it again if avoidable... More space rarely causes issues. Less space spreads things faster. So many variables...
Welcome! @EggSighted4Life has listed the standard recommendations, but as mentioned, it just depends.
Too little space is really not good, so build bigger if at all possible! Having a walk-in coop is best too, for your health, and ease in managing it.
If you add your approximate location to your profile, that helps too.
Mary