Currently, R7RS (tracking R5RS) does not constrain the sizes of exact integers: in principle, the maximum exact integer supported by an implementation could be as small as 7 (though that would seriously limit the maximum sizes of strings, vectors, and bytevectors as well). R6RS requires systems to support "practically unlimited" size exact integers. It also requires that a subset of these exist, called fixnums, which must support at least the range -223 to 223-1. (Supposedly this range was chosen because it is the range of fixnums in MIT Scheme, and all other practical Schemes have larger ranges for their fixnums.) I propose that we adopt this range as the minimum range of R7RS exact integers.
The immediate issue here is that a module name may contain (non-negative) exact integers as well as identifiers in R7RS. For such names to be portable, there must be a portable range of exact integers.