You're better able to determine the size of the O-ring because you have it and the cartridge with you.
Remove the O-ring. Determine the inner diameter. You can also get the ID of the O-ring by measuring the outer diameter of the cartridge groove (where the O-ring sits). also determine the thickness and OD of the O-ring.
When you have all those 3 measurements (ID, OD, thickness), then browse through any hardware store's website and look for your O-ring. Otherwise, just bring the cartridge and O-ring with you to a hardware store and find one that matches.
I did the same before for two Delta cartridges. They were fine. I only had to replace the O-rings.
Forgot to mention that this repair I did (O-ring replacement) was for a bathroom sink centerset faucet that leaked from under the handle, not for dripping from the spout opening. Another O-ring replacement fixed the leak from the base of a kitchen faucet gooseneck spout.