The dealers typically use 3/8 inch forged eye bolts. The backing plates the dealer showed me were 3/16 inch aluminum, and I think they were 3x5 inches, but might have been a bit smaller. The Tundra may require different plates though.
Really the first thing to do is figure out where you are going to put the eye bolts based on the guidance from FWC, then figure out your backing plate options from that. I used 1/4 inch thick aluminum plate purchased on Amazon. It’s strong but easy to work with. The weak link is not going to be the eye bolt or the backing plate - it’s the truck sheet metal. You need to figure out how to spread the load out one way or another. If you haven’t already, read the threads about bed cracking issues.