If the sump is below the tank, you may want bigger, mostly because if the pump shuts off for any reason, water will drain from your pond into the sump. You want to have a big enough sump to allow the room for the extra water.
If the sump is above the pond, the water would just drain back into the pond for the most part.
That said, you need a sump with enough room for bio media that will support your stock. More is always better.
I have an 80g sump on my 450g, but plan on going smaller when I build my automatic drip system. The sump will have a drain, so no matter what, it will never overflow. And the media thats in there now, will easily fit into a smaller container.
I think if I was doing this build I would just pump water from the pond up into your filter. Use a large trash can as the filter. Cut a hole in the lid to put a large filter sock into, or even multiple ones. Have the water drain through that and then have multiple stages of filtration with bio balls or scrubbies at the bottom. Then drill another hole near the bottom, stick a pvc pipe in it and have that spill back into your pond.
If you feel the need for more filtration, just add another trash can.
