Hi there.
I'll start off with the most important thing to look out for when buying drumsticks, avoid AHEAD drumsticks at all cost. Brands to look out for are Vic Firth, Vater, and possibly Promark (I've never actually used them before, though).
Anyway, just go to the music store and test out the sticks individually. Start off with 5As, as someone has mentioned. It is the size that feels "right" to most people, and the thickness fits most hands snugly. No need to get all technical with the measurements of the length and thickness, just look out for the tip (nylon/wood, acorn shaped, tear-drop shaped, etc), the thickness, and the weight. The length doesn't vary much to a point that it is noticable when playing, and try to ignore the wood material for the time being (just avoid any drumsticks that aren't made from wood).
Ultimately, it is just choosing sticks that feel comfortable. Make sure the sticks feel like extensions to your arms, and not something you can barely grip onto. It also takes time for a pair of sticks to "break in" and feel right. So for the first few months of playing, it's normal to buy and experiment with various types of drumsticks before finding a really suitable one.