3/8" classic style, chrome covered brass.
These can be challenging for some to install. Many times the thickness of chrome on either this spout or the threads on the portafilter (or both), varies, and getting the spout to thread on may require more than one attempt. If it will not install far enough, a wrench and a very strong twist should position it over. In some extreme cases the only option is to clamp the spout in a bench vice and crank the handle. If it threads on too far PTFE plumbers tape or a gasket may be required.