Backyard Aquaponics
A place to discuss aquaponics

i found this near me
http://www.digagg.com/products/horticultural/

they have 2 sizes, which would you recommend? - from all i've read here so far, it seems to me that the larger one would be better as far as being a little easier to use, but wouldn't the smaller size provide more surface area?

The smaller size will get blocked up very quickly. Go the bigger, even though that is a bit small too. A good average size is 14mm (9/16) still plenty of surface area.

I use river gravel that they call 1/2 inch. That really just means what will fit through the 1/2" screen and it often contains smaller stuff too. The river gravel is fairly smooth and I still get good bio-filtration.

Remember that the media filled grow beds provide more than just the surface area for the bio-filter, they also provide solids filtration as well as space/support for plant roots while the flood and drain action brings aeration to everything in the grow beds. If the media is too small then between the media compacting and the solids building up, the flow through the media can become restricted.

That media with the rough surface will provide more surface area than smooth river rocks so you could go quite large on that media before you have any issues with running short on bio-filter surface area.