'A' sizes are a proportioned paper sizing system where the next size down is exactly half of the current size.
The format is an oblong

• A7 = (A6 cut in half) 74mm x 105mm
  (approx 3" x 4")
• A6 = #99537100 - 148mm x 105mm
  (approx 5.75" x 4")
• A5 = #99537200 - 148mm x 210mm
  (approx 5.75" x 8")
• A4 = #99537300 - 210mm x 297mm
  (approx 8" x 11.5")
• A3 = #99538150 - 420mm x 297mm
  (approx 16" x 11.5")
• A2 = #99538225 - 420mm x 594mm
  (approx 16" x 23")

• A1 = #99538325 - 840mm x 594mm
  (approx 32" x 23")