Hello, from what I can see you already have full lips. It is natural to have your lower lip be slightly larger than your top lip. If you desires a change in your lips size, I would recommend consultation with a surgeon familiar with lip augmentation procedures and fillers. Good Luck.
Not even sure why you are considering this. It is anatomically CORRECT for the upper lip to be thinner than the lower lip. Put another way, the fact that your "lower lip are slightly bigger" is NORMAL. No fillers for you, in my opinion :) Be well.
You should start out with maybe 1cc of filler and go from there. You do not want to overdo too much filler in the lips. It also depends on the look you are going for too. You can always add more filler to the lips if needed. Just remember if you are going to do your lips and you do not want to bruise you should not take any aspirins or anything that will thin the blood since brusing can occur.