Is their a general rule for ring sizing for line size or spinning and casting rods? I'm building freshwater spinning and casting rods. Thanks

They're are many different ways and concepts of choosing what guide sizes and how many guides you want on your fishing rod. My suggestion for you is to go on mudhole.com and look at the guide kits. You can buy the kit if you're not completely sure what to get or you can look in the description of what guide sizes they use. This way you can actually visually see what the guides will look like compared to each other and choose the right setup for you.

Jason,
Look in the library, it's in there.

Also consult Anglers Resource catalog.

Jason,
I think that generally speaking, virtually any lb test line will go through some of the smallest guides on the market.

However, if you have to worry about knots, then the guides need to be larger.

If you are building a spinning rod and plan to use heavy mono line, the guide needs to be larger to avoid issues with the line loops jamming the guide.

But certainly the other posts give very good sources for more details.

Be safe