These trousers are. big

Correct version: These trousers are very big.

Reason:

• After the linking verb are, you place an adverb before the adjective: very big.
• The period shouldn’t come between are and the adjective.

Nuances:

• These trousers are very big. (general)
• These trousers are too big. (larger than what you want/need)
• These trousers are really big. (strong emphasis)

Alternative phrasing:

• These trousers are large.
• These pants are very big. (American English)

If you want, give me another sentence and I’ll help you tighten it up.