Hello Chroma,
The purpose of this minimum size parameter is intended exactly for setting the minimum size of the buffer allocated. Thus, if you specify a size of 13 for the minimum size, any smaller strings will in fact be padded as you have seen. If you'd like to dynamically size the buffer, then do not set a minimum size for the buffer allocation. When your application runs, the buffer will be allocated dynamically based on the size of the string that is wired into the Call Library Function Node. I hope I have correctly understood your situation and that this information is useful for you. Post back if we can provide any additional assistance. Thanks.
Mike D.
National Instruments
Applications Engineer