It's soft copper and I took the cut section to the hardware store with me. It's in between 3/8" and 1/2". The shark bite/slip-on fittings are great, but they don't make them in whatever wacky size this is.
The line had frozen. The previous owner had drained the cold side and added antifreeze, but neglected to open the hot water drain. They took out the plug in the HW tank, but it wasn't enough.
All of the plumbing is copper - gas and water. REALLY old school. I really don't want to have to re-plumb the thing so I'm hoping to find the right sized coupling.