If you have smaller lips, there are some tricks you can use to make them appear larger.
The most basic thing you could do is to wear a lip plumper. I would not recommend this method if you have sensitive lips. Ingredients in lip plumpers irritate your lips which will cause them to swell. It is a possibility that these ingredients will make your lips sore so take caution when using this method. If you do decide to use a lip plumper, they are very easy to find. I rarely use a plumper so I can not recommend a good one.
The next trick you can use would be to highlight the lips. The most important spot to highlight when highlighting the lips is the cupid's bow.
In this first picture I have circled where the cupid's bow is:
Please keep in mind that the highlight here is a computer generation. Never have your highlight be that bright. Always, always blend. The point of a highlight is to draw a soft focus of light to the area.
Wearing bright lip colors also helps your lips to appear larger. Stay away from dark lip colors if you have smaller lips, darker colors will make your lips look smaller.