Simple answer? Yeah, they're sliced at the appropriate size, so you can drop full pieces of furniture, rocks, trees, etc, right into your map.
But some objects have multiple variants and are meant to be mixed and matched or arranged deliberately. There's a set of tables specifically that are sliced at 16x16 because you can customize the length and width of the table. Beds are sliced into two segments, rather than one, so you can have one part go in front of the player sprite when it moves into the bed. They're just sliced intelligently.