In order to spawn anything, you need to enable Developer Console from Game Settings.
Once you joined a game, open console by pressing "~" on your keyboard( the button above TAB) and type " sv_cheats 1" ( to enable cheats on the server)(you also need to be the admin of the server, or if you host the server).
To spawn a chicken, type ent_create chicken . You will see that a chicken is spawn where your crosshair is pointing.
To spawn a chicken by pressing a button you need to bind that comand with the key.I chosed to bind "-". To do that just type in the console bind - "ent_create chicken" . Now every time you press "-" , a chicken will spawn :D.

To increse chickens size, you need to type in the console this command: ent_fire chicken addoutput "modelscale 10" . The "10" represents the scale of the chicken. You can put every value.
If you type this command and spawn a chicken after, the chicken will be normal and you need to type the command again.

If this guide helped you out, a +rep on my PROFILE is awesome! :D