Until recently, I was only marbling small sheets of paper, about 9x12. People often asked if I could do larger sheets but my equipment prevented me from going bigger. Last time I was marbling, though, I found a container that I could use to try some larger sheets. I ruined several sheets immediately - switching from laying down a 9x12 piece of paper, to a 16x24 piece of paper, was not an instant transition! Suddenly it felt like I'd never done it before and I had to learn the technique all over again. After some practice, though, I did successfully produce several large sheets. I've started to add some of them to my Etsy shop - in two different sizes, 16x24 and 12x18.
These images are scans of the larger sheets - the scans show only a 9x12 area, though, since that's the size of my scanner bed.