the size of your filter is ultimately up to you, but if you ask around you will find that many hobbyists "over" filter their tanks. Some to a redundancy. A larger filter will be more able to help when unexpected problems arise. Also, there are tricks to maximize your filters "cleaning" ability, as well as tricks that allow you to hold more water in a tank such as a sump.
On my 55g, I use an AquaClear 110, which is recommended for up to a 110g tank. For media I use one sponge, one large bag of bio media, and a fairly large bag of crushed coral (primarily to raise my Kh,) but it also works great for biological filtration. I definately am not saying this is the best filter and combination of media but it has worked well for me for the past year and a half. Personally, I would like to change this out for a canister filter in the future for a few reasons: 1) they are argueably better filters 2) they make less surface aggitation which is good for holding in CO2 for my plants and 3) more freedom of media.
When you feel like you have a lot of fish in a tank or are possibly getting overstocked a good filter is almost a necessity (coupled with good, frequent water changes)
hope this helps a bit more