How do I measure and ensure I'm ordering the correct link length (120 vs 128)?

The 120-link CHOHO chain suits most 11-speed road bikes, while the 128-link version is standard for 12-speed builds; consult your bike's specifications or measure your old chain link-for-link to confirm. If unsure, use the derailleur method: wrap the chain around the largest chainring and largest cog, then add 2 links for proper tension.