V1[]
Indications are, LEN in SmileBasic V1 is identical to that in V2.
V2[]
LEN(string) returns the length of a string, the number of characters contained in it. It is always an integer between 0 and 256 inclusive (because there is a limitation on SmileBasic strings, they cannot be more than 256 characters long).
V3[]
Indications are, LEN in SmileBasic V3 is identical to that in V2 (except the maximum string length is 2147483648, or 2^31).