When you first pick up a pair of drumsticks, you’ll have a different idea of what feels right in the hand. It’s like a guitar plectrum, everyone has a different preference. Some people like the sturdy feel of something heavy while others prefer the light touch of a thin guitar pick. Brian May even uses a metal sixpence because he doesn’t feel like the plastic plectrums are strong enough. The same principle applies to drumsticks.
I recommend that you start by trying both a 5a and a 7a. There’s no real winner in the 5a vs 7a drumsticks match-up, but they give you a good idea of what you prefer. Most beginners like the 5a, but you might find a lighter 7a is more pleasing, allowing you to practice gentle drumming. After a while, you might turn to heavier alternatives. I recommend that you start searching on Attuned Musician for more information.