Generally, eyeliner makes your eyes look smaller. That's not necessarily a bad thing, though, because if you're going for a more dramatic, sexy look, that would work.
However, if you want your eyes to look bigger, look for products that reflect light. Something shimmery (I'd shy away from sparkles if I were you; it borders on tacky). I find that it really helps to put a shimmery shadow on your tear duct and the general area of the inner corner of your eye (sort of a v-shape, if I were to describe it. In the diagram it's outlined in red). Those places tend to reflect light most easily, and give the illusion of a bigger eye.

Sort of like that.
If you're an eyeliner addict, though, and you still want your eyes to look bigger, then placement is key. Don't line your entire eye in a thick black line - that'll definitely make your eyes look smaller. Instead, opt for a less harsh colour, like a gray or a brown, on the outer corners of your eye. Try applying eyeliner right into the roots of your eyelashes, to give the illusion of a thicker lash-line, which can make your eyes look bigger. (drawn in with red)

And sort of like that.
I'm sorry that I love Paint diagrams so much. =(