Session size for concurrent users

I have a question about codeigniter sessions size... 
Is the session stored on client side or server side?
what about size limitation? I read some topics, and some developers said 4KB and they said only session id is stored in browser. If this is true, what about concurrent users? I stored some data, like user_id, username, user_first_name, user_last_name, user_avatar_name using  set_userdata (and also some flashdata), instead of use queries in every page,  and use these values through the whole site. Now my question is about many concurrent users that want to logged in, in the same time and surf the site (like an online exam or anything). What about session size limitation? Is it considered for one user or all users?
any help?