The Seventh-day Adventist Church is the largest, worldwide Protestant church with a single membership body that is united in doctrine and has a recognized, central leadership.
You can find a list of the world's largest religious bodies at Adherents.com. There are a few larger Protestant churches such as the Evangelical Church in Germany (EKD) which are not global, and a couple of larger global Protestant churches that are not centrally governed, thus their status as a single religious body is debatable. The Anglican Communion and the Assemblies of God, for example, are composed of a number of somewhat independent and autonomous bodies.
The Adventist Church is listed among the "Largest Highly International Religious Bodies," and is the only Protestant Christian church in the list of "Most Ubiquitous Religious Bodies."
There are a few million more Seventh-day Adventists in the world than there are Jews.