Most engineers are familiar with the limitations on the size of drop panels in two-way non-prestressed slabs.
ACI 318-14, Section 8.2.4 describes two qualifications:
(A) Depth of the drop panel must be at least ¼ of the slab thickness away from the drop panel; and
(B) The drop panel must extend 1/6 of the span length from the center of support in each direction
It is important to note that these limitations do not apply for prestressed slabs, which allows the designer tremendous flexibility in sizing drop panels to increase negative moment capacity and reduce non-prestressed reinforcement over the columns. For example, you do not need to design large drop panels that are 1/6 of the span length in each direction in order to take advantage of the increased section for flexure.
We recommend that the depth of the drop below the slab soffit be at least 4 in., which is the nominal depth of a 2 x 4 plus ¾ in. plywood sheathing (plyform). The depth of the drop should be increased if required to increased punching shear capacity. I further recommend using a minimum drop panel size of 4 x 4 ft in plan. The size of the drop should be increased if necessary to increase flexural or punching shear capacity and fit shear reinforcement within the dimensions of the drop.
If you have any questions as you size your drop panels, consult an expert!
Reference: Bondy, K., “The Use of Drop Caps to Reduce Flexural Reinforcement in Two-Way Post-Tensioned Slabs,” FAQ No. 10, Post-Tensioning Institute, Farmington Hills, MI, March 2010, 2 pp.