Don't count on a 24" process lens covering 16x20. The fuji 600 compact is said to be able to cover 16x20.
calculation using 48 degrees coverage and 25.6" diagonal coverage requirement gives a minimum focal length of approximately 29". So go for a 30 inch lens minimum. This is no movements at infinity too. Be conservative. Don't skimp on coverage. There is no point in going big if your lens will crop to a smaller format with dark corners.
perhaps look for some longer protar 7 cells that are "less than pristine" if you're on a budget. If you're "on the cheep" you can't to wrong with one of reinhold's lenses in my opiinion. Get the 1000mm one if you go that route--be conservative.