You might look for the 2 gallon size Ziploc brand. I have not seen this size in any store brands. Be aware that not all stores carry the 2 gallon size, so you might have to hunt around a little, but you can get larger items in them (I know a beach towel will fit, I've used them on float trips).
Another Ziploc that is a handy size is the large "EZ Fill", I don't know the gallon capacity, but they have a pleat or gusset at the bottom that allows them to stand up.....this extra pleat allows them to expand out more than the regular ziplocs, and the tall size allows for longer items to fit. When we wnet to Animal Kingdom, I didn't want to walk around with wet shoes and socks all day after riding Kali River Rapid, so we packed sand shoes and changed into those before the ride. After the ride, I used the tall EZ Fill ziplocs to store the wet sand shoes in my backpack (I had 1 pair Men's size 10 & a pair of kids shoes in the bag , and there was room to spare....I think at least one more adult and kid sized pairs could have fit in).
One last thing, I happened to notice at Target, in the sporting goods area, near the camping supplies, they had a rack at the end of an aisle that had super-sized Ziploc brand bags. They came in 2 sizes, I don't remember the sizes, but I think one was 8 gallon. They are marketing them for camping and also for storing things in a garage or basement.....to give you an idea of their capacity, they show one of them with a sleeping bag zipped up inside of it.