Just declare the array to be the maximum expected size and use a separate int variable to indicate how many array elements are actually in use.
I guess this is the easiest way, right? Won't this reserve memory that eventually might not be used? Is it very difficult to have flexibility in the memory you need over time? It looks like this goes more low level and I'm mainly comfortable with the Arduino language that's covered in its website's reference (and of course, not with all the stuff there).