If your WC is the same as my B, that tubing should be 1/8" OD copper. That makes the ID about 1/16". You might want to check some auto parts suppliers and ask for copper tubing to hook up mechanical oil pressure gages. They use that size. You will find a lot of nylon tubing for that purpose but I have also seen copper. It's a kit that comes with some adapters and the correct size ferrules.
The tubing is also referred to as ACR tubing. Thats for air conditioning and refrigeration tube. It is soft copper that is flexible. If you can't find that 1/8" OD size locally, this site is an on-line source. If you poke around on the site, they also can supply the correct size ferrules (also called sleeves).
If you continue to have difficulty finding this tubing it may be tempting to replace it with more readily available 1/4" OD tubing. I do NOT recommend that. If your WC is like my B, that small tubing acts as a restrictor that meters oil flow to the rocker assembly (and the governor also on my B). If you decrease the flow resistance of the tubing by increasing the size greater than the original design, you may increase oil flow to the head area and decrease it to the engine lower end bearings. That's not a good thing IMO.