While you're logged in, press the 'profile' button on the top right of the page. From the profile page, use the 'edit profile' link on the bottom left. In the profile editing page, there is an option called: "I have a big screen and want to see a larger puzzle panel".
Can't it be enlarged any further? It could still be doubled in both length and heigt and it would still fit easily on my screen. Now I still have difficulties seeing some of the puzzles well.
I agree with ruluhulu. My screen ain't that big but some puzzles are so large (the Nonogram #986 comes to mind) that I'd prefer having it even more zoomed in even if I had to scroll vertically.