I've written at least one earlier posting about using objects of a known size (like a person) in a landscape photograph to reveal the true scale of a landscape. Without a sense of scale it's impossible to tell the true vastness of certain kinds of landscapes--mountains, deserts, etc. But you can also use objects of known size as comparisons to help reveal the scale of other kinds of scenes.
In this shot of the Costa Allegra cruise ship that I shot near Fort Lauderdale, Florida, for example, I used the two smaller boats to help reveal the size of the ship. You don't have to know the precise size of the smaller boats to appreciate just how big the cruise ship is because we're all fairly familiar with the approximate size of a fishing boat or small yacht.
If I had included a rowboat or kayak in the shot, however, you would have had an even better idea of all the sizes of all the boats (and if you look closely at the small boat on the left, you can see people sitting up in the bridge and they provide yet another level of scale). Still, just seeing the two smaller boats helps to give you a handle on the scale of the scene. The funny thing is that this is a relatively small cruise ship and if you saw it next to an aircraft carrier, for example, it would look as small as the yachts look in this shot. All sizes are relative! And that's the point of including objects of familiar size--they help you to make quick visual size comparisons.