The standard NGK narrowband sensors used on the TVRs are a 17mm hex and are inside a 26mm bore (so that's how small your socket has to be). That's a common size on TVRs but not a standard size for other brands.
If you're grinding a slot down the socket you need to bear in mind that the wires have high temp silicon insulation which is very soft and easily damaged - you must make sure there are no sharp edges.