Yeah, for big-big art you'd have to work in pieces or get a larger sheet of metal, but for ≈ legal size or smaller, at $1.25 a large metal sheet pan will fit the whole thing just fine.
When my table was constructed I had the carpenter put a strip of metal across the top so that I could hold stuff up there. I couldn’t believe how long it took me to realize I could magnetize the whole surface! The sheet of metal cost maybe $40? It’s totally worth it.
It’s not really been a problem so far. I do give them an occasional wipe though. All of that said, I’m not known particularly for the “cleanliness” of my work.