In our current 1.3 rules set the shield construction rules are a little before the weapon construction rules. On p.80 of the ARB (Alliance Rule Book) it's required that, "All edges of the shield must be padded with at least 5/8 inch thick pipe insulation. Bolts or protuberances are not allowed and the longest dimension of a shield cannot exceed 36 inches. The maximum area of a shield is 531 square inches (a 26 inch diameter circle)."
However there was a clarification from the ARC (Alliance Rules Committee) that changed the requirements to: "All shields must have safe non-striking surfaces and the longest dimension of a shield cannot exceed 36 inches. The maximum area of a shield is 531 square inches (a 26 inch diameter circle)."
Now with all that said, we are in pre-release for our update to the 2.0 rules system. It's not in release state yet, but many chapters are running events designed to heavily test the new system. In the new system the size requirements of shields are loosening. If you follow the above guidelines it will pass in 1.3 or 2.0 pre-release games, but in 2.0 larger shields will be possible. If you can, I'd reach out to the staff of the chapter you're looking to play in for more guidance as they will be able to help you more directly. This is because the ultimate determination is made by your local safety or weapon marshals. Of note though, with Alliance being lightest touch combat, very light construction shields are the norm. While you could have a metal shield that is padded with pipe foam on the edges, it wouldn't really be good for you in our combat system.
I hope that helps!
