cafeteria
In Italian, 'mensa' is used to refer to a place where meals are provided, especially in an institutional context like in a school, college, or a workplace. It's similar to the English 'cafeteria'. Note, however, that a more general term for restaurant in Italian is 'ristorante'.
The school canteen is very big.
In this sentence, 'mensa' is referred to a place in a school where meals are served. The adjective 'grande' describes the size of this school canteen.
The food at the canteen is delicious.
In this instance, 'mensa' is used to refer to a place where meals are provided, typically in a workplace or educational institution. The phrase describes the quality of food served in the canteen.
I have never eaten in the university canteen.
This sentence uses 'mensa' to refer to a university canteen. 'Mangiato' is the past participle of the verb 'mangiare' (to eat), used here in the negated 'mai' (never) form of the present perfect tense.