If you put 50 adult Koi into the pond then it is too small. If you put one goldfish it is not too big since there is no such concept, but the filter isn't needed to keep the goldfish alive.
Pond size and filter size have no relation. Fish load to filter size is the calculation you need and that is only a rough guess. And filter size is defined by surface area not gallons. Skippys can have all kinds of media inside. Bacteria conversion of ammonia depends on lots of issues so there's no formula.
Understand first what is exactly being filtered. In a Skippy that is conversion of ammonia through the cycle. That's it. It doesn't remove muck, poo, or anything else at least at any level that could be noticed. There's no "balance", "natural" or any other meaningless buzz word.
Understand this bacteria will grow on most any surface. Liner, pipes, pump, falls, stream and that the bacteria will multiple as food (ammonia) increases until there is no more space or conditions allow. For almost all water gardens the surface area of the pond is enough without any filter. Koi ponds are different. True Koi ponds, not just water gardens with a Koi or two. A bunch of 20+lb fish being fed pounds of food a day in a small pond has to run like a waste water treatment plant.
Then you will totally get this whole filter thing. Of course if you just like filters, which I really did when I first started, then you should build as many filters as big as you like. You should have fun with the hobby. To much filter won't hurt anything other than maybe your pocket book and free time.
There is absolutely only one way to determine if a filter is large enough, by measuring ammonia levels. There is no way anyone can correctly tell you if any filter is large enough. I mean, pretty much everyone will, but they will also tell you what stocks to buy and which horses to bet on too.
There are of course some good guesses based on past experience. A 70 gallon Skippy for 800 gal pond would normally be considered huge. Skippys are kind of by definition in the 50-150 gal range. Other sizes using the same scheme just have different names.
For me, in that size pond, I would put something like 10-30 feeder size goldfish, no filter at all, a 100-200 GPH pump, small stream and waterfall, monitor ammonia at the start and as the fish grow, buffer pH and call it a day. But that's me.
Cool to see someone from Rochester. I grew up in Kendall, 30 miles west.