Greece was the centre of Toynbee’s intellectual world, but he was 70 when he published his first book on it, 80 when he published a second.
Hellenism, The History of a Civilization, OUP, Home University Library, 1959 (short)
Some Problems of Greek History, OUP, 1969 (over 500 pages and ultra-specialised for the most part)
Constantine Porphyrogenitus and His World, OUP, 1973 (nearly 800 pages)
The Greeks and Their Heritages, OUP, 1981 (nearly 400 pages, posthumous, finished in 1974 and published in the year of Greece’s EU accession)
Hellenism, of course, covers Rome too. Constantine Porphyrogenitus belongs to Byzantine, not classical, studies. The Greeks and Their Heritages takes us forward into modern times. Some Problems of Greek History is the only book entirely about ancient Greece.
His Roman book also touches on Greece:
Hannibal’s Legacy, The Hannibalic War’s Effects on Roman Life
Vol I: Rome and Her Neighbours before Hannibal’s Entry
Vol II: Rome and Her Neighbours after Hannibal’s Exit
OUP, 1965 (about 1,400 pages)
You could drop Hellenism as being a mere introduction and speak of a quartet of large-scale Greek or Roman works published from the age of 76, with the first book really about his main subject published at the age of 80.
Although Some Problems of Greek History is long, the description “large-scale” perhaps does not even suit such a technical and minutely specialised series of essays.
The masterpiece, by all accounts, is Hannibal’s Legacy, although its thesis (link to a good summary) is controversial. Toynbee intended it as his monument alongside A Study of History.
Constantine Porphyrogenitus is a great achievement marred by comparatively weak treatment of the Byzantine church.
Having been a backpacker in Greece for nearly a year, he should probably have produced a modern Pausanias.