Calculates the size of ANSI string
#include <WinAPIMisc.au3>
_WinAPI_StringLenA ( Const ByRef $tString )
| $tString | String Structure to process | 
| Success: | String length in characters | 
| Failure: | 0 | 
Returned length does not include terminating null character.
Search lstrlenA in MSDN Library.
#include <MsgBoxConstants.au3>
#include <WinAPIMisc.au3>
_Example()
Func _Example()
    ; Make the string buffer. It's "char" type structure. Choosing the size of 64 characters.
    Local $tStringBuffer = DllStructCreate("char Data[64]")
    ; Fill it with some data
    DllStructSetData($tStringBuffer, "Data", "Callipygian")
    MsgBox($MB_SYSTEMMODAL, "_WinAPI_StringLenA", "Length of a string inside the buffer is " & _WinAPI_StringLenA($tStringBuffer) & " characters.")
EndFunc   ;==>_Example