PCUSA is much larger (about 1.7 million), but has been losing membership for 50 years. Currently losing about 5% a year.
...what about the Westminster Presbyterians...I heard they’re growing as well...
A micro denomination that's statistically irrelevant. Their website shows two congregations at this point. They adhere to the original Westminster Standards. Non-instrumental exclusive psalmolgy but aren't in fellowship (unless something has recently changed) with others that hold that distinctive. http://www.wpcus.org/congregations.asp
Since at one point they may have shrunk to one congregation, if they are back up to two, I guess you can say that they are fast growing, doubling in size.
There is the recently formed and rapidly growing "Evangelical Covenant Order of Presbyterians" which is made up of PC-USA "dropout" congregations which decided they've had enough of trashing the New Testament..
http://eco-pres.org/