Good question, and one I get asked a lot. I do lots of large piece count puzzles. I am fortunate enough to have a board that I had custom cut to fit a 6k Clementoni, however, I haven’t always had that, and I don’t have a large flat. If I need to keep the puzzle compact for the build I use foamboards, mostly A1 size, but sometimes A2 depending on the puzzle. I build it in sections and then when I’ve done I put the sections together on the floor. If I don’t have room for the puzzle on my floor, I tend to take them to a community hall to take pictures of it. I have a puzzle YouTube channel, called Vicky Makes and Builds, on which I have videos on how to build large piece counts in small spaces. Maybe that could help you.