I don't know if there is a general rule for length. I like the chain to hang about half way between the ground and frame.
If your going to buy new chains, mock it up with a light piece of twine, to get the idle length. A little longer or shorter is no big deal. Remember to cross the chains.
My OEM chains were a bit too short for me. They would bind on tight turns. I used plastic chain to mock up a new set at the length I wanted using "add a links" as the temp connection point. To gauge the right length I set my truck and trailer up in a pretty tight turn and measured from there. I cut the old ones off the frame and welded on the new set.
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2022 33RBTS
Progressive Industries EMS - Hardwired
Equalizer 4 Point WDH
2021 Ford F350 7.3
Air Lift Rear Bags w/ On Board Compressor (Pending)
2016 28BHBE - (Traded and Missed already)
If your chains are too long all you need to do is put a few twists in the chain to suck up the slack, if you’ve ever rented a uhaul trailer, their chains are all long, they just twist them to fit each specific tow vehicle