As humans we perceive three dimensions because we have binocular vision. Our camera sees only in monocular therefore we need to (as photographers) use perspective to show depth.
To portray size or scale of an object be it large or small, introduce something into the composition that is of familiar size (telephone pole, sign, a person or an animal).
To suggest depth also place within the composition something of familiar size (vehicle, chair, a person or an animal) at different distances. A good example of this is rail road tracks, the merging of the tracks suggest that there is depth and dimension to the photo.