In Poland, when you order from pizza chains (as opposed to regular restaurants, which tend to have overpriced, undersized and undertopped "italian" pizzas), the "large" size circles around 40cm in diameter (which Google tells me is around 15.7"), cut into 8 pieces. Thickness varies, but you usually don't get very thin pizzas unless you ask specially for that option.

14" is closer to medium size, which depending on earlier meals may just about satisfy two adults, or leave them hungry. With four adults, I can only see it leaving them wanting another pizza.