Used three chains on all my similar sized efforts. Chain is 1/2" pitch so a typical 116 link chain = 58". If you have the trike close to finished you can run a tape around where the chain will go to get a close idea. You can use split links or just join the chain with a chain tool.
Popshot - exactly the information I was looking for. Much appreciated. Still a few weeks from completion, working on Steering & brakes now. But I want to have all the "purchased" components on hand so I have them when I'm ready for them and I don't have to order and wait.
3 chains will do the job and you will have a short piece left over. For me KMC chains are preferred, The master link is very nice to use. I usually keep at least 3 new chains on my parts shelf, the local bike shop only had Shimano chains the last time I went in. the master link they use isn,t as user friendly. Get yourself a chain tool. Something like a park tool - 1.2 master link tool. Way better than messing around with long nose pliers.
I may be teaching granny to suck eggs but make sure you buy multispeed chain, assuming you're using a regular rear derailleur. Single speed chain (1/8" width) is too fat to use on a multispeed (3/32" width) rear end. Also there are differing multispeed chains which all essentially appear the same but those destined for 9, 10, 11 speed etc applications are thinner in the chain plates as these rear cassettes pack the rings closer so make sure you buy something compatible with the number of speeds. If you intend to add a potent mid drive (front mounted on a Warrior) motor at any point the chain is also going to have to transfer that power so don't buy the cheapest. A reasonably fit rider may put out 250W or so, a top athlete 400W, a Bafang mid drive motor 1200W or more at peak.