Another look at the supposed McMansion comeback has a pretty simple definition of McMansions:

The go-go days of the late 90s and early 2000’s gave us the McMansion, those 5,000-square-foot homes decorated to the hilt.

This keeps McMansions at the simplest level. To start, they are big homes. With the average new home at 2,500 square feet, 5,000 square feet is double the size. Second, I think “decorated to the hilt” refers not to the interior decor but rather to the garish or impressive features of the home such as large entryways, roofs with many gables, and a whole range of stone/stucco/wrought iron/glitzy features on the front.