The fire riser room is not required to be locked
Access
Automatic sprinkler system riser, fire pump and controllers shall be provided with ready access. 
Where located in a fire pump room or automatic sprinkler system riser room, 
the door shall be permitted to be locked provided that the key is available at all times.
 (IBC 2018 902.1.1)

You do not need a door in the fire riser room that opens directly to the outside

You only need a door that opens directly to the outside if you have a fire pump in that room. 
The IBC states:
Except as provided in 4.13.2.1.1.1, 
fire pump rooms not directly accessible from the outside shall be accessible
 through an enclosed passageway from an enclosed stairway or exterior exit.

9 ft² per system valve is typically enough space for your fire riser room
Every project is unique, true, but is there any rule of thumb on space needs? 
In my experience, at a minimum leave at least 9 ft2 space per fire 
riser system valve while maintaining at least 3 ft of space in front of
 the fire riser that will always remain open and free from storage.