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Places that sell safety chain should have a chart that shows chain size and strength. Rule of thumb, you want EACH chain to have a breaking strength at least equal to the Gross Trailer Weight of your trailer, whatever that happens to be. This gives you a 2-to-1 factor of safety when both are hooked up.
As for length, that's a little tougher to determine. When they're hooked up, both should be about the same length, and neither should touch the ground, but otherwise they should hang at least halfway to the ground at the lowest point of the catenary (that's the shape of a sagging chain), to make sure you've got enough slack in them for turns.
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