What is process measurement assurance and do small labs need it?

It scales down well and is arguably MORE valuable for a small lab, because an annual external calibration alone tells you nothing about the other 364 days.

Minimum viable program per SOP 30 / GLP 1: pick one stable check standard per measurement area, measure it with your normal process weekly or per batch, plot on a control chart with limits from your accumulated standard deviation, act on out-of-control points (investigate before releasing results). Time cost: minutes per week.

What it buys you: drift detection between calibrations, evidence for auditors that results stayed in control since the last calibration (limits the recall scope if something fails as-found), and a live Type A estimate feeding your uncertainty budget. ISO/IEC 17025 (7.7.1) effectively expects it — 'monitoring the validity of results'.