20" is a great size! Jreeper, would you mind posting a photo of your fish with a ruler or tape measure next to it? That could help us update Cat-eLog on Planet Catfish.
There seems to be no consensus on the max size of Clarias batrachus.
SeriouslyFish says what Jreeper cited above: 40″ (100cm). However this is rarely achieved in captivity, with 18-24″ (45-60cm) being considered a good size.
FishBase, PlanetCatfish, and Wikipedia are all consistent among themselves and cite a much smaller max length:
***** FB: Max length : 47.0 cm (18.5") TL male/unsexed; (Ref. 40637); common length : 26.3 cm TL male/unsexed; (Ref. 12193); max. published weight: 1.2 kg
http://www.fishbase.org/summary/Clarias-batrachus.html
I am not sure what TL means in FB's definitions.
***** PCF 15.7" SL (standard length, that is excluding the caudal fin - this definition I know)
http://www.planetcatfish.com/common/species.php?species_id=90
***** Wikipedia cites FishBase: Walking catfish has an elongated body shape, and reaches almost 0.5 m (1.6 ft) in length and 1.2 kg (2.6 lb) in weight
http://en.wikipedia.org/wiki/Walking_catfish
***** Smithsonian Marine Station: Walking catfish typically attain a standard length of 225-300 mm, although animals twice that size are encountered (Courtenay and Miley 1975; Hensley and Courtenay 1980).
http://www.sms.si.edu/irlspec/Clarias_batrachus.htm
***** FLMNH Ichthyology Department: Reproductively mature at one year of age, this species is reported to reach a length of 61cm (24 inches) in its native range, although the largest individuals in South Florida rarely exceed 36cm (14 inches). Overall, little is known regarding the age and growth of this species.
http://www.flmnh.ufl.edu/fish/Gallery/Descript/walkingcatfish/walkingcatfish.html
***** USGS: Size: 61 cm in native range; rarely.
http://nas.er.usgs.gov/queries/factsheet.aspx?SpeciesID=486
SHOULD BE NOTED: Clarias is a big genus with lots of similar-looking species. A lot of these inconsistencies may be due to mis-ID.