The lighter, the better, up to a point. You still need enough space to be comfortable at night. I'm 6'4". I had a Sierra Designs Clip Flashlight 2 that was very light, and packed small, but I couldn't quite stretch out in comfort. Then I found the Microlight 2 from L. L. Bean. It's just a little roomier than the Clip Flashlight, and not much heavier. It has proven to be perfect for me. It's big enough to be comfortable, there's enough room for me to have some clothes and stuff next to me when I sleep (I have Ortliebs so I leave them outside.) It's lightweight, and it doesn't leak.

For someone who's not so tall you could get by with something a little smaller. I love the Seedhouse 2, but it's just a few inches shorter than I'd like.