The size of a food web is limited by the number of

It is limited by the number of primary producers (ie photosynthesizes such as plankton (marine ecosystems) or grass (land-based ecosystems) but is ultimately affected by things like weather. For example, if a certain area is experiences a drought, then there wont be enough water to feed the photosynthesizes, which will affect the primary consumers, and the secondary consumers, and ultimately the Tertiary consumers.