You just have to find an integer type with the largest storage class, cast the value to it, and then use the appropriate format string for the larger type. Note this solution will work for any type (ptrdiff_t, etc.), not just size_t.
What you want to use is uintmax_t and the format macro PRIuMAX. For Visual C++, you are going to need to download c99-compatible stdint.h and inttypes.h headers, because Microsoft doesn't provide them.
Also see
http://www.embedded.com/columns/technicalinsights/204700432
This article corrects the mistakes in the article quoted by Frederico.
