Do you have a smaller size?
'Piccola' — PIC-co-la. Double 'c' — crisp sound.
Use when the item is too large. This is the companion to 'avete una taglia più grande?' — both are essential shopping phrases.
'Più piccola' is the comparative — 'smaller'. The article 'una' is used because you are asking for any available smaller size, not a specific one. If the M is too big: 'avete la S?' is also natural.
Questa è un po' grande — avete la S?
This is a bit big — do you have the S?
Combines problem and solution.
La taglia inferiore c'è?
Is the next size down available?
More formal phrasing.
Mi serve qualcosa di più aderente.
I need something more fitted.
About fit rather than size — for structured clothing.
Italian ready-to-wear sizing ('prêt-à-porter') often runs slightly large in the hips and small in the shoulders due to traditional Italian body proportions in pattern making. It is common to need different sizes for different brands.