This is very simple to answer, and I suggest that you look at various images of lips to appreciate the different facets.
1. Symmetry between the left and right halves
2. Ethnically appropriate proportions of the upper and lower lips
3. Age-appropriate volume
4. Defined boundaries (sharp vermillion borders)
5. Pronounced cupid’s bow (central part of the upper lips)
6. Central fullness (median roundedness, tapering laterally)
7. Up-turned corners
8. Some visibility of the upper ‘wet-dry border
9. Surrounding changes, such as fine lines around the lips, contribute to a general older look.
Importantly, contrary to the trend in recent years, bigger is not necessarily better!