What it says:
Quirk: Fire Cracker - It gives him the ability to rip off chunks of his hair and explode them like firecrackers. The blasts are strong enough to destroy buildings, but according to Balugo, they're "pathetic."
Given Buildings it plural, this means the explosions are either powerful enough to destroy several buildings, or pack the firepower to destroy most types of buildings where they are in the world. This means we shouldn't take the smallest buildings there available. Average-sized buildings or even large buildings should work if it were to be calculated.