This link suggests that a board that's 16 x 12" might be good.

Our largest cutting board is 14 x 10".

Some cutting boards around that size:

• Dexas — 18 x 12 x ½" — The reviews say it warps?
• Dexas — 16 x 12 — ⅝" thick.
• KitchenAid — 18 x 12 — ⅝" thick.
• Martha Stewart — 16 x 12 — ½" thick — Has handle.
• Norpro — 16 x 12 — ¼" thick, which is extremely thin.

I confirmed that our washing machine would fit a cutting board that's either 16 x 12" or 18 x 12".

Others:

• Farberware — 18 x 12 — ¼" thick, which is pretty thin.
• Belwares — 18 x 12 — ⅓" thick
  • It comes in colors like blue, green, and red. (likely for kosher households)
• 18 x 12 x 1/2
• 18 x 12 x 1/2
• 18 x 12 x 1/2
• 18 x 12 x 1/2