The size of the prey which can be consumed would depend on two features of the digestive tract. One is the size of the oral opening since this would limit the size of prey which could be taken into the digestive tract, at least the cross-sectional area of prey though long prey with a small cross sectional area might be taken into a long digestive tract. The length of the digestive tract could also be limiting, since the capacity of the digestive tract is limited by its total volume, which is determined by both cross-sectional area and length.
Work Step by Step
Use an analogy: a pipe, which is like a digestive tract. Think of what limits what you can put into a pipe and have it completely within the pipe: the diameter of the pipe (determines cross-sectional area) and the total length of the pipe.