Let's do a back-of-the-envelope calculation. A mole of gold has a mass of about 197 grams, in the ballpark of our hypothetical deck of cards. The lattice constant of gold is about 0.4 nm (in a fcc structure, but let's assume simple cubic for simplicity). A commenter mentions a thickness of 2 atoms, but that's a very fancy setup, so let's go for something that's feasible with modern techniques currently applied in industry: 10 atoms. Arrange our tenth of a mole into a square and we get sides of a bit more than 2*10^(11) atoms, corresponding to a square with 80m sides. Actually not that far off from a tennis court - I would have guessed it would be bigger.