Starting with Ver6, there is no distinction between full-width and half-width characters. In Ver5, for full-width characters, you had to specify Num as two characters, but from V6 onwards, both full-width and half-width characters can be counted as one character.
 
Example)

If you want to get "あa" in Ver5
var a = ::LeftStr("あabc", 3);

If you want to get "あa" in Ver6
var a = ::LeftStr("あabc", 2);