"What weight are you?"
Translation:Cén meáchan atá tú?
Your answer is the most common phrasing in Munster. Cén isn't really used there by traditional speakers, and cad é is generally used in its place. Additionally, while there are sentences like Táim trí chloch déag meáchain with no prepostion, in all of the (Munster) sources I consulted i was more commonly used e.g. Cad é an mheáchaint atá ann - "How much does it way?" (meáchaint is the Munster variant of meáchan). So I'd say this answer should definitely be accepted.