Depending on your size, i would choose between 20, 24, 26.
If you are into street but still into hucking off stairs, i suggest getting a single speed street/ DJ bike such as an eastern night train, norco 250, or an eastern traildigger (the traildiggers come in 20, 24, and 26).
All the bikes above have front suspension (besides the traildigger).
I suggest you have front suspension if you want to do a little hucking. But keep in mind, street/ DJ bikes won't hold up for to long if you focus on mainly involving stair hucking as your main riding style.
If you want to do a bit of huckering i suggest one of these...