Very impressive project. I admire your persistence! Well done! If you did 20x24 you wouldn't need as many sheets, by the way, and would line up with the final dimension almost perfectly.
The biggest trays I could use were 16x20". It's also the largest sheets I've ever printed on, and I was constantly worried about creasing the print (I only had 10 sheets — 9 to use, 1 for testing exposure).