As best I can tell, Barna's reference to "the average church" includes all denominations.
"The Barna Research Group says in the United States the average church attracts 89 adults on a typical weekend...Barna says only two percent of churches have more than 1,000 adults in a typical weekend." -- From Small churches heartbeat of Convention by Norman Jameson
I would think that "the average church" is likely smaller than the average person tends to think it is.
All of the churches I've been associated with would be below average. We thought churches with 89 in attendance were large churches!