First off, you know to sleep at an angle, right? If I lay in line with the ropes I get back pain too. If you lay oblique, you can approach a much flatter lie, but you need a much longer hammock if you're a tall guy in order to bring that off.
Your preferred length is going to depend on a lot of things, actually, and I haven't found a specific source for "this height and weight = this ideal hammock length" data. A gathered-end hammock will need to be longer than a version with spreader bars - a caternary cut in the gathered ends will help prevent the ridge in the middle. The angle you hang at matters a great deal too. Around 30* at the tree is ideal - more, and you can't get sideways enough. Less, and you end up creating nasty stresses on your ropes or straps and you won't be as stable either. Even with all that, there's still some personal preference involved, so experimentation is necessary.
If you can't make an ideal hang angle, an adjustable ridgeline can bring the ends of your hammock closer together to give it the right sag, and I've found that getting that sag right is crucial. It took me a couple nights to find my ideal "hangle", and it didn't help that I'm a side sleeper and shift positions a lot.
Here's our camp at the last QTD:
The blue hammock on the left is a Grand Trunk Single. I've slept several nights in it, but I feel like it's too small for me. My wife is 5'6", and is learning to like it. The green hammock on the right is home brew - it's a 90x132 taffeta tablecloth in a classic gathered end style, and I sleep much better in that one (I'm 6'4"), because the greater distance between the ends gives me more room to find the right oblique line to lie flat (it's actually way too wide, and probably too long, but better too much than too little). The next time I feel like messing with it, I'm going to put a caternary cut in the ends that'll go a long way towards eliminating the ridge that develops in the middle of the hammock and likes to dig into your calves and cut off circulation to your feet. Then I'll start trimming material off the ends and the sides until I trim too much, then I'll know my ideal length and I'll either buy a commercial hammock in the right length or I'll get some good ripstop and make a better/lighter one. The white one in front is my boy's, and it's another homebrew that I've put almost zero effort into because he'd sleep on a bed of cactus when he's camping and still wake up refreshed.
If you suffer from Cold Butt Syndrome, consider putting a sleeping pad in the bottom of your hammock for insulation, or buying/making an underquilt to hang underneath it and prevent heat loss. I use my sleeping pad, and my only grip is it likes to twist out from under me when I roll over, so I'm considering building a pocket to keep it in place.
So far we only break out the hammocks when the weather promises to behave, I haven't even started weatherproofing our setup yet. That's a whole 'nuther bucket of fish.
What I haven't learned from trial and error I've picked up from
www.hammockforums.net, it's a great resource if you're serious about making a hammock work. I'm loving mine - the whole setup is lighter and packs smaller than most tents out there, and since I beat the learning curve I sleep much better hanging than I ever did on the ground. If I had a way to conveniently hang a hammock in my bedroom, I totally would.
