Auto insert specific value if condition met

I'm new in postgreSql. I have to create a database a venue. The client book a room for an event. I'm supposed to have 2 values for each room, their capacity of people it can hold(stand and seat). When a client book for a room, I'm not supposed to input the room capacity. It has to check which room have been inserted and assign it automatically. I'm thinking about some kind of if statement or CASE but it has to assign and not select. Do u guys have any advice for this problem. PS: There is only 4 rooms(Main room , Side bar, meeting room 1 & 2) Thanks