I have tried many pore reducers (mine aren't large, I just want a smoother look), and I have found nothing better than Dr. Brandt Pores No More. I use it as a primer and my makeup looks smooth and lasts that way all day. Also, it's not just a cover, but actually shrinks pores and absorbs oil. A little goes a long way.
Important to note, when you've come to the "last" of the product, cut the top of the tube open and reseal with one of those big paper pinch-type-clips... there's still a lot of product in it (my only complaint is that they redesign the pump)!
I've also found some powders to accentuate pores- I use Smashbox HALO and it does not do that.
I would look at Dr. Brandt's Pore Refiner, which smoothes and perfects skin, reducing the appearance of pores: http://www.sephora.com/browse/product.jhtml?id=P13
A lower cost option is Clinique Pore Minimizer Instant Perfector, which is also a great product: http://www.sephora.com/browse/product.jhtml?id=P12
I would check out the reviews (there are quite a few available) to determine which one meets your needs.
NOTE: These products only reduce the appearance of large pores. Since it's impossible to change the size of your pores, you can only make them look smaller by keeping them clear of dirt and oil and using a product to disguise them.