Dynamic dimensions in Bus object

Hi, I'm developing a sink C-MEX S-Function which handles a Bus object with four crucial fields as input. The first three are fixed-sized doubles named min, max and step. The fourth field has to be a double array whose dimension is defined by ceil((double) ((max-min)/step)). In my C-Struct I simply make this field a pointer which I can initialise dynamically with the values of the first three fields of the first incoming bus signal. How do I have to configure the bus object to accomplish a dynamic dimension of the fourth field in my bus object?

Thank you very much in advance!