Any roof less then 10,000 sq. ft I use 2 roof drains ANYTHING over 10,000 Sq. ft I use 4 roof drains.
Now for sizing these roof drains I take the pitch of the roof area and figure my fixture unit valve as the more steep the roof the greater the flow rate AND I have to know the type of roofing membrane and if we are going to use a controlled flow system as not to over load the drainage system in case of a heavy deluge.
I also like redundancy by the use of scuppers no more the 4″ above the roof and located near the drain area if possible.
About the actual size of the drain leader line I personally like sizing the piping full size for the roof area so in case one or more drain should fail I know that any one drain can handle the volume.
Now you did not state if this is a combined system like soil and waste and storm all going into the same system as found in older cities.
If you are going to use a combined system you have to take all the fixtures connected and by interpolation either figure everything as storm sized Sq. ft or soil and waste as fixture units.
It is also a great idea to get a 10 year average flow rate during the raining season and the load factor of the roof so you can keep ponding to a minimum by over sizing the drain if needed.
When sizing the drain inlet you should know if this is a weather protection roof Or a promenade deck?
Why don’t you send me an E mail and we can work out exactly what number and size of drainage would fit your exact requirements.
Here is a web site that may help answer some questions as to why the plumber should size the drain piping and the Plumbing contractor should be held personally responsible if anything is found to be improperly designed.
The plumber should be 100% accountable and thus he/she should be the only person to write these specifications.
By the way If you do contact me ILL take full responsibility for any of my designs and specifications and even give you a certificate of insurance if requested (for a consulting fee).