I use Postal Annex. I don't know exactly how "big" you are talking about, but I got a QuickSet shelter from Amazon, and Postal Annex accepted it just fine. Box was 7 feet long, and probably about 12" each side.
You don't have to rent a postal box from them - although if you're going to be in an area for several months, it's probably cheaper. Otherwise, you pay a fee for each package you pick up.
Just call the location you'd like to use, and ask them how to address the package. I imagine just about any place that offers private mail boxes would do the same.