20 sq. feet is pretty small. I doubt any deer would want to jump in an area that small. Thats less than 5 ft by 5ft. You could barely fit one deer int there. Deer don't want to feel like they are trapped and with an area that small they would surely feel uneasy. Id say you'd have to go at least 100 sq. feet, thats an area 10 x 10 and I still think that would be too small. Id keep extending your fence until you start to see tracks in the fenced in area.