How do you use this thing???

Ok, as most of you know, I joined CURVES over the summer and started on the road to fitness. Well, I'm doing pretty well, but I've still got a lot of work to do. Almost everytime I go to a store I'm looking for work out clothes. But on my latest quest, I've been looking for one of those giant exercise balls.

Well, while out Christmas shopping for the kids today, I found one...and bought it. Now I would be amiss if I said I knew anything about exercise equipment. They had this ball in 3 different sizes 55cm, 65cm and 75cm. Again, I haven't a CLUE as to which one I need. So, I did what any woman would do and I chose the pretty red one (75cm) :-). Of COURSE I wanted it in purple, but Nooooo, they didn't have any. Little did I know that each size had it's own color. So we get to the counter to pay and I ask what's the difference in the sizes. The gentleman tells me that it depends on your height. He glances over the counter and tells me that I'll need the medium sized one (65cm). Well, you have NO idea how excited I was to be able to purchase a MEDIUM sized ANYTHING for myself. But wouldn't it be my luck that the medium size ball is BLUE! Oh well...I'll get the blue one.

So, I get this giant GLOBE home and proceed to inflate it. You know what? Rubber has an AWFUL smell! I get it all full of air and now I'm gradually being convinced that I should've gotten the larger one. Not because it's red...but because it just seems like the larger one would be better for me.

Better for me HOW you ask? That brings me to the reason for this post...How do you use this thing??? Sure, it came with a DVD and a little wall poster. But I don't get it!!!
So, if any of my fellow readers have EVER in life used one of these things, please help me out! I'd love to know if in fact I should've gotten the bigger - 75cm (prettier) red one. Or if my height is a factor. AND even more so, what are the best exercises to do on this thing. I'm looking forward to using it as I think it would be great for when I'm not able to go to the gym.

Thanks in advance for ANY suggestions!