Excerpt from [the DustJS tutorial](https://github.com/linkedin/dustjs/wiki/Dust-Tutorial#size_keyxxx___size_helper_Available_in_Dust_V11_release):

    Array - number of elements, [1,2,3,4] has size=4
    String - length of the string, "abcdef" has size=6
    Object - Number of properties in the object, {a:4, b:8, c:15, d:16} has size=4
    Number - Value of the number, 23 has size 23 and 3.14 has size 3.14
    Undefined, 0, empty string - zero
    Any other value - length after conversion to string