To create illusion of a smaller mouth, use lip liner and draw the line on the inside of the lip line.
To create illusion of a larger mouth, use lip liner and draw the line slightly outside the lip line.
For the best look, always aim for a liner that matches your lipstick as closely as possible. Never use lip liner that is many shades darker than your lipstick.
Dark, matte shades will make lips look thinner, while light, glossy, shimmery shades make lips look fuller.
When you want to make your lips look larger, don't go far from their natural edges, because trick can look too obvious and it will not work. Remember, you want to enhance your lips, not create new ones!
Choose your lipstick colors wisely. It is all about you wearing the color, not the color wearing you, so pay special attention to lipstick color.
If you dab a bit of lip gloss over the lipstick, you will add dimension and make your lips look fuller.