Perfect symmetry unfortunately isn’t possible.  Everyone is born with a bit of discrepancy in size from one side of their bodies to the other.  The goal is to make it appear as evenly matched as you can so that the discrepancies aren’t as noticeable.
Differently sized implants can help with some unevenness, but for small asymmetries it might be better to go with two implants of the same size instead.  I suggest this because there are some asymmetries in the implants themselves when you choose two different sizes, which could prove problematic in reaching your goals.

Also, when you augment breasts that have an existing size asymmetry the % in the difference between the breasts will decrease even with implants of the same size.  This is because the over-all size of the breasts is increased, which makes the differences in appearance less noticeable.

I’d suggest consulting with a plastic surgeon experienced in correcting asymmetrical breasts and decide your best recourse based on that.  He or she will be able to examine you in person and help you decide the optimal way to achieve the results you’re hoping for.