Cast the size to uint64_t to make it a known size

Not all platforms have the format specifier to print size_t

Change-Id: Iad3d15867e2d6953f274e51617223af4502f80c2
Reviewed-on: http://review.couchbase.org/31198
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>