I think the biggest bag I have is the Monogram Cabas Piano Bag. It's actually probably not that big compared to other tPFers but the bigger my bag gets the more I try to stuff in it!!!
The biggest bag I own is the Neverfull MM and I love it since it is so light. I thought I didn't like larger bags but after the Neverfull I would definitely consider another big bag.
my biggest are the Manhattan GM, Batignolles Horizontal, Mahina XL and Embossed Polly. i think since the Galliera is kind of hobo-shaped you would be able to pull it off, like the Mahina