Aggie's got the right idea--it doesn't have to be that complicated. You just need alot of room that you can be messy in--slosh chemistry around etc. Some folks will dilute developers down more than normal and then sponge these on by hand in sections etc.
The place where i work has had at least 2 darkrooms over the years--our new one, is actually designed to make murals in--very long for horizontal projection with an alcove for a roller processor. We've only made maybe 3 mural sized prints in this though, since we send that work out now. We did these using a 20 inch roller transport processor and tiling strips of mural paper together. These were last-ditch, final hour efforts because of someone's (sigh) poor planning...needless to say those involved didn't enjoy that day at work...
The old darkroom was a monster as well--There were 2 5x9 foot sized kreonite sinks arranged in an L pattern with a fume hood above them. The sinks had standpipes & water panels and water recirculating jets built into the corners. You could fill them up with chemistry to process the paper, and then turn them into print washers, or use one for a water rinse prior to the fix.
They chewed up a good 25-30 gallons of chemistry for each sink, and disposal of the fix was problematic which is one of the main reasons they stopped making prints this large in-house. Plus it was a time-consuming PIA....
What they did was to build risers alongside the sinks, so you could walk back & forth to agitiate the print--at about knee height. They also had some home-made tubes that were big sections of PVC--and they swirled the paper around in these things to agitate. My boss tells me they pretty much sucked..I only made "large" prints in this old lab using trays that were in the 3x3 foot range. You can get trays up to 40 inches, or just under 40 but they cost quite a bit....
At any rate, it just depends on how much you plan on doing this. If it's just now & then, you can cobble almost any setup together--if it's more than that, you might want to consider paying a lab to do it....in almost every way, it's worth it (to me anyways).
Hope this helps--
guess my employers would want me to say these are my opinions only.