validationcaches: Use size_t for sizes

Summary:
...also move the 0-clamping logic to ApplyArgsManOptions, where it belongs.

This concludes backport of core#25527
https://github.com/bitcoin/bitcoin/pull/25527/commits/0f3a2532c38074dd9789d1c4c667db6ca46ff0ab

Depends on D15378

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15740