Do you have one size up?
'In su' is spoken as two quick words: 'in SU' with stress on su. Don't run them together.
After trying something that's too small. Italian sizing can run small in some brands, especially tailoring. Always worth asking.
'Una taglia in su' literally means 'one size upward'. 'In giù' would be 'down'. This is an idiomatic expression that can't be translated word-for-word.
Ha una taglia in giù?
Do you have one size down?
When the item is too large
Ce l'ha più grande?
Do you have it bigger?
Simple and direct alternative
Questa è un po' stretta. Ha qualcosa di più largo?
This is a bit tight. Do you have something wider?
More descriptive explanation
Italian tailoring distinguishes carefully between 'taglia' (overall size) and fit at specific points like 'spalle' (shoulders), 'vita' (waist), 'fianchi' (hips). Alterations are common and often offered in-store.