MCrooks, in the Apo-Nikkor line 900 mm won't cover 20x20. You need a 1210. In Apo-Ronars, a 1000 will just cover 20x20, a 1200 will just cover 20x24. Goerz claims a 42" Apo-Artar will just cover 20x24 at infinity. All info from the makers. AFIK no 900 mm process lens will do what you want. My 900/10 Apo-Saphir certainly won't, according to Boyer it covers 655 mm at infinity. johnielvis highly desired 32" Apo Ronar won't do for you either.

None of these lenses is available off the shelf, the best source is eBay. You'll need a lot of patience and a lot of money and considerable luck.

I'm with CJ. If you want to do it, go for it, but make sure you understand what you're in for. Don't use rule of thumb to determine focal length needed, check the makers' claims. The rule of thumb you used to decide that 900 mm is long enough took you over a cliff.

Shuttering a huge lens shouldn't be a problem for you because wet plate emulsions are so slow. I hang a Compound #5 in front of my little 900, don't think that this solution (or an Ilex #5) will be good on a 1200/9. But it will work on a 1200/14. And Packard will make a huge shutter for you, to be hung at one end of the lens, that will be perfectly good for time exposures.