The most relative example of Scale in an interior is furniture. Furniture should be the appropriate scale for the size of a room. If you have a huge room, dainty furniture is going to look silly and dwarfish. You could also apply it to wall decor. If you have a huge empty wall, a small scaled painting will look really out of place by itself.
The ottoman and chair are the appropriate scale for each other, but look oddly small next to the bed and side table.
There are ways to play with scale though, and that's where Mass comes into play. Mass is the actual or perceived weight, density, or relative solidity of a form. If you have a smaller scale couch, you can actually make it appear more massive by covering it in a darker color (and vice versa). A glass coffee table will appear lighter scaled then a solid coffee table of the same size. Furniture on legs will appear less massive then furniture that is solid to the floor.
The above table's glass top, thin legs, and light color make it appear light in scale and less massive, as opposed to the dark, solid wood, and bulkier table below.
Massing is another strategy to create the illusion of larger scale. Massing is when you group furniture or objects together to create more mass. If you don't have a large picture for a large wall, you could group some pictures or objects together, and together they will appear to be the appropriate scale. If you have a small chair and a couch, you can group the chair with a side table and lamp, to give it a more appropriate scale.
If you feel like you don't have an eye for appropriate scale or mass, just take time to observe store displays, model homes, etc. Compare the actual scale of objects to their surroundings, and notice how colors affect their perceived mass. Then play in your home. Also, sometimes taking pictures of arrangements and looking at the picture is easier to analyze than the arrangement itself.