refactor: extract STATIC_SIZE constant to prevector

Summary:
Co-authored-by: Anthony Towns <aj@erisian.com.au>

This is a partial backport of core#32279
https://github.com/bitcoin/bitcoin/pull/32279/commits/756da2a994c5f6cf552f025bab5d60dae0582b09

Test Plan: ninja all check-all bench-bitcoin

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18813