What size socket to remove rag joint nut on '92 LX?

(Borrowed pic)

Are you talking about the 12 point bolt that's in this picture? If so, it holds that U shaped clamp on the splined shaft of the rack. I don't know the size but as mentioned 12mm is close. It can be tough to break loose where the splines are. Spray some PB Blaster in the joint where the splines are. After sitting long enough to penetrate into the fitting you can tap it upward with a hammer and heavy chisel or whatever you have. The rack will have to be moved forward as it comes out because it only goes upward a little bit. Hope I'm describing what you need.