How powerful was the Tibetan Empire during it's peak?

I honestly don't know how to answer such a general question like this. The Tibetan Empire waxed and waned between Songtsen Gampo's initiation of the Empire in the 640s and Ralpacan's assasination in 836. During Songtsen Gampo's reign, Tibetan armies invaded India to avenge the dishonor from one of Harsha's successors onto a Chinese envoy. They also vassalized Nepal along the way, returning Songtsen Gampo's father in law to the throne of Nepal. About a century later Tibetan armies allied with the Arabs defeated a Chinese army at Talas River, a domino that would end with the An Lushan Rebellion, and a ten year siege which ended with the Tibetan control of Dunhuang and the Silk Road. At which point the Tibetan Armies sacked Chang'an, installed their own emperor, and took tribute of a collapsing T'ang Dynasty.

This period of Trisong Detsen's reign seems (to me) like the height of the Tibetan Empire, but it is Ralpacan who is remembered for both being the most devout Tibetan Emperor and the most militarily victorious, though I can't specify which exact conquests he is allegedly responsible for. And within five years of his death, the empire collapsed and was claimed by two of his nephews (well, they were infants, really it was the consorts of his elder brother who ran two rival regimes), while rival clans resumed their domination of various territories and the Tibetan Dark Age began.

I'm not really how to rank power as the most I can give you is more general examples of Tibetan Imperial reach. Things like GDP aren't really applicable in this era. But that's not really it's "peak," more like the entire rise and fall of the "Empire." Prior to the empire it was more like an internally focused and bickering kingdom. Afterwards it was a free-for-all.