Hey Alex,
Well, it depends on what your strings contains: Simple ASCII, UTF8 or UTF16.
And it's not that trivial in any of the UTF(X) cuz under UTF8 not all chars are 2 bytes.
I think the unit LazUTF8 or LazUTF8Utils, something like that, has utilities to calculate these values.
But in the end, I think the usual Length(AString) will give you a value you can then use.
But again, you'll have to make sure there is no Unicode magic influencing that count.
Cheers,
Gus