Initial Beam depth depends on factors like

1. The span by depth ratio as per IS 456:2000 cl. 23.2.1 (depending on the different cases). we can add 50 mm in addition to that to get the initial depth for beams. 
   
2. The available formwork sizes and the sizes considered for that project (to reduce the number of different sized beams).
   
3. The beam-column capacity ratio works on the concept that beams should have a lesser capacity as compared to columns to achieve a strong column and weak beam arrangement. The beam to column capacity ratio should be lesser than 1 to achieve such a condition.