There's no single right answer that will cover any 20' sloop, the size and shape of the keel the form of the hull and how the sails are ballanced will all affect what size rudder is needed, as well as the shape of it. Also the answer will depend on what handling characteristics are desired.
As for just the depth of the rudder, usually it will go to slightly less depth than the keel when the keel is at it's deepest. Some lifting keel yachts, especially smaller ones, also have a lifting rudder.
The Sloop Inn was created in 1312.
It is a "sloop", said to be "sloop-rigged".
Dorothy Sloop was born in 1913.
Kingfisher - sloop - happened in 1864.
Dorothy Sloop died in 1998.
Greek sloop Karteria was created in 1826.
A Sloop has only one mast a schooner has two.
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Sloop John B was created on 1966-03-21.
It's from the Dutch word: sloep. (Which, as far as I can tell, means "sloop.")
Texan sloop-of-war Austin was created on 1840-01-05.
Texan sloop-of-war Austin ended on 1846-05-11.