It doesn't suit me / It doesn't fit me.
Flowing phrase — say it smoothly: non-mi-va-BE-ne.
Use when clothes do not fit well or an item is not right for you, without specifying a defect. Good for size or style issues. Less formal than 'non è della taglia giusta'.
'Andare bene' literally means 'to go well' but in Italian it means 'to suit', 'to be right', or 'to fit'. The indirect pronoun 'mi' (to me) personalises it. This is one of the most versatile expressions in everyday Italian.
Non è della mia taglia.
It's not my size.
More specific — about size.
Non mi piace come sta.
I don't like how it looks on me.
About the look, not the fit.
Non è quello che cercavo.
It's not what I was looking for.
Vague but polite — implies a change of mind.
Italian shops often prefer offering an exchange (cambio) over a cash refund, especially for clothing. Knowing your exact Italian size before shopping saves much back-and-forth.