One common use of the term McMansion is simply a large home. In this blog post about the mortgage interest deduction, the writer contrasts the price of McMansions to more normal-sized homes:

That means average homeowners with modest Capes and fixer-uppers are helping subsidize others stretching to keep up with the Jones and their million-dollar McMansions.

The measuring stick of a McMansion in this post is how large the mortgage is:

A close look at the interest rate deduction reveals much of its benefits go to homeowners with mortgages far larger than most in the middle of the housing pack. Check out this Forbes piece, which nicely lays out the argument for taking away this perk from the homeowners with outsized mortgages – incredibly the limit is currently $1 million…

The president’s deficit commission recommended capping the deduction’s use at $500,000 in mortgage debt, down from $1 million now, while nixing its use for vacation homes and converting what’s left to a 12.5 percent tax credit.

OK, I vote for keeping it simple and just lowering the mortgage cap to $500,000 or $600,000, while making second homes ineligible as well.

So a McMansion here would start with homes that cost $500,000 to $600,000. In most suburban communities, this buys a large home. In denser areas, not necessarily. What about older homes that cost this much – are these McMansions? It wouldn’t take too much searching online of real estate listings to translate these prices into square footage in particular areas.

Overall, this use of the term McMansion seems to refer to any large house beyond “modest Capes and fixer-uppers.” This use of the term seems quite vague: a McMansion is any (presumably larger) house above a certain price point.