I am not a designer but i can tell you a basic concept behind it.
Owner specifies, my beams should not pop up from wall face - this becomes a width parameter which you can have max.
Regarding Depth of the beam, you can have max depth as much as it wont obstruct the other services, such as window - here you can have max beam depth is above lintel level.
Lift - here you can have max depth as per the height of lift doors (this is most critical thing as the heights of lift door manufactured by all companies are not same. and breaking beam to adjust the lift in lift shaft may cause a severe damage to structure).
So while designing you can keep a customer point and serviceability issues on mind for deciding the height and width of the beam.
Rest you can always test your structure theory as per dimensions you decided to use.