> So it's really FD_SETSIZE that matters, and it's a compile-time constant.

The only way this constant can matter is if the array has to end with an empty slot, which doesn't appear to be the case. The pre-compiled function can't tell how you compiled the code calling it, and the only place where FD_SETSIZE is used is in the macros to set them.

Dynamic allocation ought to be just fine, and likely faster for big arrays.