Revert "use long long instead of size_t make it more safe"

This reverts commit 2c75f2cf1aa0d9cc8256517bd6f3d8a82088ee1a.

After further analysis, it is very unlikely that we'll raise the
string size limit to > 512MB, and at the same time such big strings
will be used in 32 bit systems.

Better to revert to size_t so that 32 bit processors will not be
forced to use a 64 bit counter in normal operations, that is currently
completely useless.