The standard O2 sensor hex is 22 millimeters. Various designs abound, but the socket I bought, a Snap-On, doesn't fit the early Bosch sensors! Others look like a big crows-foot end with a 1/2 inch drive. They work well too.
Splicing is best done with solder, and then use the heat shrink. Crimping is not the best way by itself. The NTK sensors come with the wax lined heat shrink crimp connectors to exclude water, but the wire is best connected with solder, and then use a regular heat shrink. (two layers if you are fussy)