After getting a denture top lip is smaller. What's the best option?

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.

There are many good fillers you need a formal consultation to determine what is best.  I suggest seeing an expert for an evaluation.  Best, Dr. Emer.

Most people will lose upper lip vermilion show when getting a denture due to loss of natural teeth support and maxillary bone resorption. While injectable filers may be beneficial for a temporary improvement, they can also create a duck lip look if the vermilion is too thin. In very thin upper lips, a lip advancement is a permanent method of recreating more visible vermilion show.

Congratulations on taking control of your lips.  I prefer hyaluronic acid fillers for the lips and of those, juvederm ultra plus is my favorite.  It is the smoothest and least painful to inject. It looks the most natural and gives the longest lasting results of an HA in my opinion.  Lip proportion is also important.  Most agree the lower lip should be 1.6 times larger than the upper lip.  Hope this helps.  Stay skin

It is difficult to comment without photos, however, filler can add volume to an atrophic or under projected lip