It's actually more like 1.3 million. Still a good size though. About as big as Albuquerque.
Bigger than Albuquerque, Tucson, El Paso, Fresno, Bakersfield, Chihuahua City, or Mexicali (the other metros in the same size range as TX's RGV in the border states region of US and Mexico).
The RGV consists of 4 counties in Texas; Starr, Willacy, Cameron, and Hidalgo counties. That is two metropolitan statistical areas centered around McAllen and another around Brownsville/Harlingen.
https://en.wikipedia.org/wiki/List_of_primary_statistical_areas_of_the_United_States
This is their population as of 2014.
RGV
McAllen-Edinburg, TX Combined Statistical Area: 894,028
Brownsville-Harlingen-Raymondville, TX Combined Statistical Area: 442,295
Total RGV: 1,336,323
1. Rio Grande Valley, TX: 1,336,323
2. Albuquerque-Santa Fe-Las Vegas, NM Combined Statistical Area: 1,165,798
3. Fresno-Madera, CA Combined Statistical Area: 1,120,522
4. Mexicali-Calexico, CA-MEX: 1,102,342
5. Tucson-Nogales, AZ Combined Statistical Area: 1,051,211
6. El Paso-Las Cruces, TX-NM Combined Statistical Area: 1,050,374
7. Chihuahua City: 958,000
8. Bakersfield, CA Metropolitan Statistical Area: 874,589
If you include the parts of RGV directly across the border in Mexico (Reynosa and Matamoros) from the parts of RGV in TX and combine it, then its just slightly less than 3 million people. Over 1.6 million people live in Reynosa-Matamoros across the border from McAllen-Brownsville-Harlingen/Rio Grande Valley in TX which has over 1.3 million people, combined with its Mexico side thats just a little less than 3 million people.
Rio Grande Valley, TX-MEX: 2,948,323
TX has another metro like this with the El Paso-Ciudad Juarez-Las Cruces area too, which has 2.8 million people. In Cali they have San Diego-Tijuana which has 5.1 million people and Mexicali-Calexico which has more than 1.1 million people.
The RGV and El Paso metros when you take into account the TX and Mexico sides combined are actually larger than ATX or SA metros.
Laredo is another one that is growing fast both sides of the border and should be the next big one over 1 million. BTW, the RGV side in Mexico is the fastest growing area in all of North America. So it will continue to grow at breakneck speed. Laredo-Nuevo Laredo had 775k people in 2010, by now its probably close to 850k and heading towards 1 million too.