This is very simple to answer, and I suggest that you look at various images of lips to appreciate the different facets.
- Symmetry between the left and right halves;
- Ethnically appropriate proportions of the upper and lower lips;
- Age-appropriate volume;
- Defined boundaries (sharp vermillion borders);
- Pronounced cupid’s bow (central part of the upper lips);
- Central fullness (median roundedness, tapering laterally);
- Up-turned corners;
- Some visibility of the upper ‘wet-dry border’;
- 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!