Yes, anyone can do some form of yoga regardless of size, flexibility, etc. It's about working with what you've got and paying attention to what's going on in your body more than it is about being able to reach a certain distance, fold up in a certain way, etc. It's challenging, yes, but for me that just gives me incentive to keep trying. And, as others have mentioned, its about much more than just the stretching, balance, etc. More than anything else I've done, it really strengthens your mind-body connection, and can clear your mind better than anything else. After a while, if you pay attention and stay mindful, you can tune into that clarity and calmness without being in class. You can also, of course, make use of certain poses during the day for a quick break, and you'll be amazed at how refreshed it can make you.
I also recommend a class when you're first starting; there are all kinds of different flavors of yoga, some more vigorous than others. You may have to shop around until you find a style, teacher, pace, etc. that suit you. (My favorite style, and one I think is especially suited to those of us who are out of shape, is Anusara, but it's not a terribly common style.) If you look into classes at general-exercise type gyms, I would find out what qualifications the teacher has; a lot of "yoga teachers" in gyms are aerobics instructors who don't really LIVE yoga. You want to take classes from a real yogi.
If you still decide to go the video route, I'd suggest the
Yoga for Weight Loss DVD from Gaiam --
www.gaiam.com. It's not for weight loss in the calorie-burning sense, but in the sense that the instructor understands that a lot of overweight people have lost the connection between their minds and bodies, and that, of course, some posses are more difficult for us. You'll also want some props; Gaiam also sells a kit that comes with a video that shows you how to modify poses with the props to make them more accessable to you.
Another thing you might think of doing, at least to start, is to pick up a couple of books by Miriam Austin --
Yoga for Wimps and
Cool Yoga Tricks. Wimps demonstrates sort of a pre-yoga poses. The approach the shape of some classic poses, but heavily propped and modified for people who think even a beginning yoga class might be difficult. The second book is for people who are taking a class or using videos, but would like some deeper explanations about some standard poses as well as modifications and variations.