What size legal pad should I buy?

The standard is 8.5 x 11 or 8.5 x 11.75 inches, which is what most picks here use. A compact 5 x 8 pad like the Oxford small pads is better for pockets, clipboards, and quick lists. Note that some sellers vary sheet count (30 vs 50 vs 100), so check sheets-per-pad against the price before assuming a deal.