1) A Modern understanding: "Big Box" refers to the collection of discrete, independently existing individuals who happen to be engaged in a similar set of beliefs and/or practices, at generally the same time and place. However, their unity is not "authentic," only assumed or imposed. That is, they do not participate in some universal greater than themselves, so that they are a particular instance of that universal, but instead, having rejected the idea of universals they constitute themselves as a "unity."
This is why they need to "brand" and "market" themselves: branding and marketing are "external" ways of establishing identity, unity, and "relationship." (For Modern churches, they are perhaps the only ways.) Modern "big box" churches are "plants" in the sense of factories. They offer a product, and if one can assemble the necessary "parts" and master the necessary techniques, more of the product can be delivered.
Thus, the Modern "big box" church is located at a specific space-time coordinate composed of a certain quantity of members located at that coordinate, as opposed to the postmodern "no-box" church, which resists definition even in terms of specific space-time coordinates. In general, postmoderns react strongly against quantification, calculation and prediction--and so reject the "programs" approach of modern Big Box churches, while still maintaining the modernist metaphysical commitment to discrete individuals
2) A premodern understanding of "big box" might refer to the particular instance of the universal. That is, the particular thing participates in or images the greater thing, so that its existence is dependent upon the reality of the universal. It does not constitute itself as a collection of individuals, but rather receives its existence as a result of its relationship to the universal, and is one of many possibile instances of the universal thing. Its definition is therefore impossible apart from the universal.
The premodern church is a "box" insofar as it incarnates the universal: it is a concrete, physical instance of the Body of Christ. It has a nature which is given to us to understand, enjoy and instantiate. It may be big or small, but it has a specific location in time and space, and most importantly, it has a history. From a postmodern perspective, boxes are seen as confining: they limit the individual's freedom. Hence, the more the premodern church recognizes a history and honors a tradition, the bigger the box it appears to be.