It would be cool to have an automatic tool to generate a blob of any size and it would include as much detail as possible within your given blob size limit...
pmtiles extract INPUT.pmtiles OUTPUT.pmtiles --bbox=BBOX --maxzoom=Z --dry-run
--dry-run will output the total size of the final archive without downloading any tiles, so you can adjust the maxzoom from the default of 15 (for basemaps) until it fits under your limit