I have a 450mm Nikkkor M and a 600mm Fujinon-C that have a circle of illumination large enough for the 20X24" coverage. But to get acceptable definition on the edges of the field you must stop down to f/64. Even them the far corners are just a bit soft.
Then I have the 550/11 XXL, which covers 20X24 with some movement.
I also have a 30" Red Dot Artar, which covers 20X24 nicely, but again you need to stop down to f/32 or f/45for good performance on the far eddges.