Sizing an Unknown-Scope Migration

You're asked to use a dynamic workflow to migrate every usage of a deprecated internal logging library to its replacement, across a codebase you don't know well. You genuinely don't know whether this touches 20 files or 400 — nobody on the team has counted, and the old library was used inconsistently over several years.

1. Walk through, step by step, how you'd use the size guideline setting and the slice-first habit together to get from "no idea how big this is" to "confidently running the full migration," without either quietly undershooting coverage or blowing an unnecessary amount of budget finding out.
2. Explain what the Large workflow warning would and wouldn't tell you in this scenario, and why it's advisory rather than something you should treat as a stop sign.
3. Once you know the real scope, explain how you'd decide between medium (the default) and explicitly setting large or unrestricted, referencing the actual agent-count thresholds.