The trousers are too long.
'Pantaloni' — 'pan-ta-LO-ni'. Four syllables, stress on third. Don't rush the middle syllables.
Very common situation — ready-to-wear trousers are cut for average heights. In Italy, shortening trousers ('accorciare') is a standard and inexpensive service.
'I pantaloni sono' — plural subject needs 'sono'. 'Troppo lunghi' — 'lunghi' is masculine plural (pantaloni is masculine). Italian adjectives must agree in gender and number.
Potete accorciarli?
Can you shorten them?
'Accorciarli' = to shorten them — 'li' replaces 'i pantaloni'
Quanti centimetri devo togliere?
How many centimetres do I need to take off?
Asking for measurement guidance
Li posso portare già domani?
Can I bring them in tomorrow?
Discussing timing for the alteration
Italians always measure trouser length with the actual shoes the trousers will be worn with. Different heel heights dramatically change the correct length. Bringing your shoes to a trouser fitting is standard practice.