Pass size_t to span functions: services/

In preparation for upgrading various span args from size_t to
StrictNumeric<size_t>, this fixes type mismatches that would cause
compile errors with that change.

Bug: none
Change-Id: I71d4996035938a3bbb8251d92bbba4e4682a82dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6020233
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Reviewed-by: David Schinazi <dschinazi@chromium.org>
Commit-Queue: David Schinazi <dschinazi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1382906}