Switch to size_t to avoid conversion warnings from clang++-17

[eddelbuettel]

Issue #1299 alerts to issues from the (very much opt-in, not (yet ?) compulsory) conversion warnings clang++17 can do. As I noticed a few from one of our own tests a first attempt squashed those.

This likely does not 'close' the issue so not adding the tag, but should do no harm.

Checklist

• Code compiles correctly
• R CMD check still passes all tests
• Preferably, new tests were added which fail without the change
• Document the changes by file in ChangeLog

[kevinushey]

LGTM!