How do I pick the size of my Tennis Racquet?

I'm a beginner in playing tennis. I've always love watching the Grand Slams and smaller events on tv but I figure it would be a great fun way to start getting in shape. But when I look for tennis racquets they have sizes, I know I'm not a junior size but full size racquets have sizes too like: 4 1/2, 4 1/4, 4 3/8, 4 1/8. What do those sizes mean and how do I pick which size is best for me? I'm 5'11 and 155lbs, so I'm tall but thin. Any suggestion? I'm not looking for an expensive racquet as I will not be playing professional just for recreational purposes. I've been looking at Walmart, Target, & on Amazon.com.