According to this article, which we found helpful, a bog filter should be 10-20% the size of the pond surface area.

I don't know if it mentions the water flow, but I have always heard that a little slower flow is better than faster. Ours is probably too fast (6,000? gph pump for a 10,000 gallon pond), but it seems to work anyway. Of course there are always those people that have NO filtration and clear water, so who really knows what works... :-)

Here is a link that might be useful: bog garden construction