The North American Plate (I assume that is the one about which you are asking because I think most of us here are Americans), consists for everything from from the Mexico north and the middle of the Atlantic west to the West Coast (excluding Baja California and the sliver of Southern California west of the San Andreas Fault). It is moving westward. So, the West Cost of the continent is the leading edges of the plate. The trailing edge is at the Mid-Atlantic Ridge where new sea floor is being created. The forces involved when one plate collides with another do cause compression and deformation in the leading edge of continental masses. So, yes, in some places this is causing a decrease in the width of the continent. However, along the San Andreas fault, the North American and Pacific plates are sliding past each other. The lack of compression has actually allow the continent to rebound from the compressional forces that were once acting on that part. So, that part is actually getting wider. This is what created the mountains of the Great Basin. Additionally, as North America overrides the plates it is running into, various fragments of highlands have been getting scraped off and added to the continent. This has the affect of making the continent grow bigger. (All of Alaska is made up of the foreign terranes as they are called.) So, as you can see. The process involved on the boundaries can be rather complex and there really is no simple answer to your second question.

See page 364 in your textbook. Top left corner. . . . :)