19 feet, and you're at roughly 90 sq ft of sail. For sails that large, you're not (usually) cranking them down - you are walking down the line and then winding up the line once the kite is on the ground.
While the same type of kite in a larger size will generate more lift, it's not exactly proportionate to the increase in size. For example, more lift will usually mean a higher flying angle which usually reduces pull. For a kite with design such as a Rok the size is almost proportionate to the size based on it's design but for delta it's much less so.
You touch other issues as well, such as winding line while the kite is under power. While a winding reel will avoid several issues, it can still make a mess of the line.
Would I fly a 19' delta by hand? Nope. But holding any kite to fly it is not something I like to do. But everyone is different and should do what they like to do.