There's an option in the Object Editor to decide the shape (and thus size) of the pathing. I don't remember what it's called, but the info you put into it aren't numbers, but building size samples. This means you can choose whether it got the pathing size of an Altar (those are all the same), Barracks, a tower or even critters (they have no pathing shape, making them not have a grip and they don't 'snap'. That means they're exactly as accurate as the mouse cursor instead or snapping to the next 'square'). Just check out which building shape matches your preference.