"a big building"
The most accurate word for "Building" is بناء and مبنى. both have the same meaning when it comes to the known "Building" , but differ a bit when referring to other form of building (like in "building of the sentence").
However, بناية is used colloquially and rarely heard in MSA.
Actually, since the word building is somehow weird in English, they both can translate to building.
However, بِناية = 'the act of building', or better said 'construction'. You can use it in sentences like "My son like building stuff with Legos". On the other hand مبنى literally means 'what is built'. Therefore, I would prefer using مبنى in this sentence.