The largest I've seen are the 6 litre ones in IKEA. They're the squeeze together type though not the true zip up ones with the sliding bit on the top.
I would advise against using plastic bags for Lego storage. We used plastic tubs. But if you are looking for something for storing the smaller pieces, I have a lot of airline amenity bags in various styles and colours which might be useful. They are free and can be collected near Waldshut.
Thanks for the replies, I will check out Ikea again (I only found the smaller ones on the website).
I already have the legos in tubs, but we have 8 large tubs and there is just too many. We're looking to remake the sets, and put them in bags (and back into the tubs).