I may have missed something here, but why can’t you just declare your string with the desired size? For example, in my VBA code I often use something like:
Dim AString As String * 1024
which provides for a 1k string. Obviously, you can use whatever declaration you like within the larger limits of Excel and available memory etc.
This may be a little inefficient in some cases, and you will probably wish to use Trim(AString) like constructs to obviate any superfluous trailing blanks. Still, it easily exceeds 256 chars.