The most common layman interpretation of "large-scale art" refers to the physical size of a piece, for example the massive Abraham Lincoln in Washington, D.C., or almost any of Jeff Koons' giant balloon animals. Large doesn't always have to be in comparison to human size; it can also be something larger than an object normally would be--for example, an extremely thick book would qualify as would a thin book the size of a large toddler.
Another way to look at large scale is in terms of its ability to reach hundreds and even thousands of people. Conceptual and performance art can become large scale through the participation of people, even though they are not about creating an object but rather expressing an idea. An example of large-scale performance art would be a piece in which people in one country write "love" on edible wafers, that are then eaten by people in another country, who also prepare wafers for more people. The wafers occupy negligible physical space but the idea has reached hundreds of people.
Another way for a project to be "large scale" is to incorporate many different artists to make a piece, object or show. For example, art toys are oftentimes used for these projects. They are given to hundreds of artists who are asked to modify them in their own style. The results are then put on show. While the toys are separate objects, they remain part of the same project. An example of this using a single object would be creating a totem pole. One person would cut down the tree, trim it and carve it, another could paint, while yet another could add beading and feathers.
Sometimes, and sadly rather often in the arts, the parts are greater than the sum. When it comes to large-scale art projects this essentially means a "big"--important--artist is creating something. It can be a literally large piece but more often than not the "large" refers to the status of the artist creating the piece, not the size, significance or quality of the art.