I frequently fly metal beer signs from breweries I visit. Most are in the 20x20 inch range, with some a little bigger. The biggest "issue" I've had with them was a handful of times where the FA asked if they could store it in the closet instead of against the wall at my seat. I also had a flight on AA years ago where someone had a long object, possibly a sculpture, and the FAs were friendly with figuring out how to help the pax.