How powerful was the king of Magadha known as Dhana Nanda?

As a rule of thumb, anyone who controls Madhyadesha is already one of the Greats of the time.

Firstly, it was a full-fledged Empire. Magadha had been attempting Continental dominance for more than a millennium before any Nanda was heard of, and the Empire had already passed through three dynasties, each of whom had contributed to its growth. All three, despite leaving copious records, used to be regarded as “mythical” by our worthy Intellectuals since it's more or less accepted Academic thought that from Hitler's bunker was the Arya born.

Anyway, they've dug up enough to guarantee at least two of these pre-Nanda dynasties and, if by some miracle Hindus survive until 2050, the third will also be proved beyond doubt.

Anyway…

The Emperor Dhana was, as I've said earlier, one of the Greatest Lords of the Earth with dominions stretching over Madhyadesha, Mahakantara, Kalinga, the reaches of Samtata and Kamarupa, Banga and Anga, Chedidesha, and possibly Andhra. His family had presided over the growth of the largest burecracy ever built in Bharata and massive infrastructure development- such as the canal stretching from Mahanadi delta to the Ganga at the very least as repaired by the Emperor Kharavela four hundred years later.

The Nandas were much maligned by Scribes in the early Classical period, likely on account of Maurya influence- a situation Indian Intellectuals would usually protest but, in this case, approve of since the primary Maurya figure in our histories, the Emperor Ashoka was marked by his adherence to the teachings of Shakyamuni, a rather unorthodox preacher, while the Nandas were universally sticking to the more accepted schools of Hindu thought.

PS: The Emperor Ashoka was far more orthodox a Hindu than either you or me. The Buddha himself was far more orthodox a Hindu than either me or you. Wait for my dear friend VH's book titled “The Buddha was against Caste and Misogyny and other jokes you can tell yourself.”