If you really want to be able to adjust the fscanf format programmatically, you could try stack-allocating a string with enough space, and then generating the format like so:
e.g.
char formatString[100];
// writes "%max_size[0-9]", substituting max_size with the proper digits
sprintf(formatString, "%%%d[0-9]", MAX_SIZE);
fscanf(fp, formatString, buffer); // etc...
