When looking for a con, you have to consider what genre you'll be writing or wish to write. If it's fantasy/sci-fi then I'd use this website.
http://www.scificonventions.com/html/findevent.phpSmaller cons can be good because it does give an opportunity for more one on one contact. But larger cons may cater to more writers or have more discussion panels for new writers. Aside from meeting publishers, cons may also give you the opportunity to network with other writers.
A friend of mine who is working on her own comic book often gives out a printed sample issue to various publishers and people in related fields. It's really helped her open doors because it showcases all her talents. But it is a financial investment. And only now is it starting to pay off.
Each year, I go to Dragon*Con in Atlanta, GA. Huge con. Tons of writers. Right now, there are 41 on the guest list. By September, it will rise to about 400 (media, music, writers, artists etc...). But it can be an expensive convention to attend.