For many years I've been using a 6x18" Gold's Gym foam roller, available at Walmart and Target for about $15. They also have a 6x30" for about $24, but actually I've always found the 18" to be adequate, or even a little too wide. If you're just needing a roller for massage and/or exercise aid (yoga, etc), then 18" is probably all you need. I'm a pretty large guy, over 6 feet tall and over 200 pounds, and I actually find the 18" slightly cumbersome (probably 16 would be ideal for me). Having said that, however, I went ahead and got the 36" because I wanted to cut it up and experiment with different lengths. It cuts easily with a hacksaw or even utility knife. I wanted small, medium, and large sections, for different exercises. For example, 10-12" is perfect for the thighs, but 4" is ideal for the spine. (The spine exercise takes practice, but is perfect for back alignment. A massage ball also works). In terms of density, all the rollers I've tried (Golds brand, this Ringside brand, and others at the gym) are all comparable in density and quality. You can go to Youtube for lots of ideas and examples of how to use a roller. Here are some of my common exercises: 1) Back alignment Any sized roller can be used on the back for spinal alignment (though the narrow one works best). It won't help with your neck or lower back, but it will do a terrific job keeping your middle back aligned. When I go to the chiropractor, I never ever have problems with my middle back because of these exercises. Laying flat, I roll up and down my back, and hear it cracking into place. If you have a particular spot that's out of alignment, lay down, center the roller over that particular spot, and try to put pressure on the problem spot, arching your back with your butt still on the ground. 2) Deep Tissue - Calves If you have an injury or a particularly tight muscle group, the only thing that truly helps is deep tissue (lots of pressure). You can do much of your own deep tissue work at home with a roller. The keys are (a) go very slow, (b) to put as much pressure on the muscle group as you can stand, and (c) having a partner/spotter is always a big help (they may need to push down on the roller). But you can do it yourself. 3) Deep Tissue - Outer Thighs A partner/spotter can roll the foam on your thigh while you lay on your back or side. Or, you can watch one of the instructional videos for ideas about doing it solo. Massage therapists say that outer thighs tend to hold a lot of tension, a commonly overlooked muscle group needing massage. Good luck!
