From Second Life Wiki
StringLength
llStringLength
Function: integer llStringLength( string str );
Returns an integer that is the number of characters in str (not counting the null).
Examples
// assumptions:
// object name: LSLWiki
// script name: _lslwiki
default
{
state_entry()
{
string HowLongAmI = "123";
integer strlen = llStringLength(HowLongAmI);
llOwnerSay( "'" + HowLongAmI + "' has " +(string) strlen + " characters.");
// The owner of object LSLWiki will hear
// LSLWiki: '123' has 3 characters.
}
}