Tommy, there are a few things you need to look for when selecting a cast net. First, keep in mind net sizes are measured by radius. I would recommend starting out with a 6' radius. This size is relatively easy to throw, yet still covers enough area and sinks fairly quickly. The next thing to consider is mesh size. The bigger the mesh, the quicker the net sinks, however, a bigger mesh causes more baits to "get stuck" or escape. If most of the gizzards you are catching are less than 8'' i would go with the 1/2'' mesh size.
Check out Champion cast nets at www.memphisnet.net
or the fitec super spreader.