Do you have a smaller size?
'Piccola' — 'PIK-ko-la'. The double 'c' gives a slightly longer stop before 'o'. Three syllables.
After trying something that fits well overall but needs to go down a size. A basic and essential phrase for fitting room use.
'Una taglia più piccola' = one size smaller. 'Più piccola' is the comparative of 'piccolo'. The feminine agrees with 'taglia'. 'Ne ha una più piccola?' also works — 'ne' replaces 'di questa'.
Ne ha una più piccola?
Do you have a smaller one?
'Ne' replaces the item already mentioned — more fluent Italian
Provo la trentotto?
Shall I try the thirty-eight?
Stating the actual size you want to try
Questa è troppo grande. Esiste in misura inferiore?
This is too big. Does it come in a smaller size?
More formal — 'misura' as synonym for 'taglia'
Italian sizing, especially in women's fashion, can vary substantially between brands. A '38' in one brand may fit like a '40' in another. Always try before committing — it's expected and encouraged.