G-Shock O-ring replacement

[maxrussell]

I've been trying to replace the battery in my G-Shock 5600. I've been able to remove the backing, replace the battery. However, I'm having issues with trying to refit the O-ring which looks to somehow have expanded to be far too large for the groove into which it should fit.

What size replacement O-ring would I need to replace the existing ring? Should I use a sealant with the ring?

[jdm]

You need to measure the groove with vernier calipers. Watch silicone grease is used there, good practice but not mandatory