We have really big trees here in the PNW. I've found a couple of times that I couldn't get 8' around the tree. I got a couple of 14' and just have a little extra slack.
I like the idea of an 8' & a 14', then if you had a bigger tree than normal, it'd still work.
Another idea was from my Kammock Python strap. I might have Paul double over say 5' on one end (you'd have to pay for 10' for this section), then another 5' with a loop on the end. You could put the hitched end through the single loop, then have about 5' of various attachment points w/o worrying about stretch (aka ENO Slap Strap Pro)...