This is the 3472nd most frequent Catalan word.
grossa
"Grossa" means "big" or "large" in English, referring to a feminine singular noun.
In this sentence, 'grossa' is used as an adjective to describe the size of the house, meaning 'big'.
La casa de la cantonada és molt grossa.
The house on the corner is very big.
Here, 'grossa' is used to describe the apple, indicating its size as 'large'.
Vaig comprar una poma grossa al mercat.
I bought a large apple at the market.
The word 'grossa' here metaphorically describes the surprise as 'big' in magnitude or significance.