In the early stages of a project, we are often asked how large structural elements will be
before we have had a chance to perform the necessary computations. We have found the following rules of thumb to be useful in estimating the approximate depth of a structural member as well as reviewing the reasonableness of a design.
It is important to note, however, that the depth of a member is influenced not only by the span but by such variables as spacing, loads, support conditions, deflections, and clearance requirements, as well. The final member design should be based on computations performed by a licensed structural engineer. In ACI 318-14 code.
Depth of beam ( inches ) = Length of the beam ( ft )
If the length of the beam is 12ft equal to the Depth of the beam should be 12 inches
For more information watch this video