First, you can't change pore size. If you've got big pores, you're stuck with them. What makes them look bigger is dirt, so you have to clean your face thoroughly. I recommend Eve Lom's cleanser which is expensive at �39.95, but it lasts for ages. It's an oil-based cleanser: people with oily skin often use oil-free products which is a mistake. Dirt is attracted to oil, and people with oily skin should use oil-based products.
There isa salon treatment which helps, too - it's called Hydradermie and it's by Guinot.
Good luck.