If your string is a char array, you could just create it with a length given by the user using new, or you could may even use the vector template for it, if you use the string template, I am sure that there is a way to use variable length as well...
Sorry, I can´t really tell you how as I have nearly no knowledge of stl yet, also it already was quite handy a few times for me, but I know that there are some ways

I btw wouldn´t buy the dll but just because I would prefer to get a little bit into the whole stuff by writing such a dll on my own :P