Drumsticks are essentially backwards in sizing. The lower the number, the thicker the stick and vice versa. You may see several sizes depending on manufacturer. However, the most common stick sizes are 2, 5, and 7. Stick sizes also have letters after them to narrow down the size even further. Unlike the numbers, the further the alphabetical character is in alphabet, the thicker the stick gets.

Each manufacturer's specific dimensions may slightly vary. However, this will give you a general idea on how stick sizes differ from each other:

2B - Diameter = .595"-.635” Length = 16 1/4”
5B - Diameter = .590”-.605" Length = 16”
5A - Diameter = .540”-.570" Length = 16”
7A - Diameter = .512"-.540” Length = 15 1/2”

As mentioned above, you may also find several other sizes listed such as Extreme, Long, Rock, etc. This usually refers to a custom length of the stick. The length doesn't usually vary by more than a half inch, but you can feel the difference. Every difference in size will create a different feel, and in some cases sound.