The sized pump is going to depend on how many GPH
your drain will do. So if your overflow puts 500 GPH
of water down to your sump then you will want a pump that will put 500 GPH
of water back into the tank - taking into consideration head height loss (the higher it has to pump up, the lower the rating so you may need one rated at 950 because after pumping up 3-4' it is only at 500).
The sized pipe will depend on what your pump wants. If your pump is rated at say 500 GPH
at 4' height with 3/4" pipe, then that is what you would use because any other size would change what the pump can do.
Having fun yet?