Anything from 6' to 9' from a 2wt to a 5wt will make a good blue gill rod. For large gills I'd go with a 9' 4wt. Most folks fish a shorter rod for panfish, but a 9'er will really help keep your backcast off of the ground. That is usually a big problem with beginning casters. A 6'6" 2wt may be loads of fun for gills, but they can be hard to cast for a beginner.
