What size of compression fitting do I need?

It is important to understand the difference between

Compression Fittings Installation Instructions - Flowflex

Pro Trucking Products: 1/2 Pipe Thread X 3/4 Nylon Tubing Male

PlumbShop Straight Compression Fittings, Chrome, 5/8-in OD x 3/8

Poly Compression Fittings

Large Size Compression Fittings, Brass Fitting Supplier

Compression Fittings • MicroGroup

G&S Penrith MALE TO MALE ZINC PLATED STEEL BICONE RING

A Complete Guide to Compression Fittings

PTFE Ferrule for 1/16 Diameter Thermocouple Probe Compression

Navien NPE-2 Series Condensing Tankless Water Heaters, 50% OFF

Copper Pipe Compression Fittings Supplier