array-dimensions

(array-dimensions array)

Returns a list of the sizes of each dimension of array, at any rank: a rank-1 vector yields a one-element list, a rank-2 array a two-element list, and a rank-0 array the empty list (nil). See array-dimension for the size of a single axis and array-rank for the number of dimensions.

A string is a rank-1 array of characters, so it answers here too. The size is the array dimension — the capacity — which for a character vector with a fill pointer is larger than its length.