In this paper, we provide non-asymptotic upper bounds on the size and average size of $s$-insertion $s$-deletion balls. As a corollary, we conclude an explicit non-asymptotic lower bound on the size of optimal $s$-insertion/deletion correcting code which is a strict improvement of Levenshtein’s lower bound given in 2002. Particularly, in the case of single insertion/deletion, comparing with Levenshtein’s bound, we reduce the gap to the maximum potential lower bound by at least $1/3$.