Unless that old monk was exaggerating
The only clue you have is from a 7th century CE travelogue by Yuan Zhang (or Hiuen Tsiang, as he appeared in history textbooks of an older generation).
Going with difficulty 600 li or so, we leave the country of Tukhara, and arrive at the kingdom of Fan-yen-na (Bamiyan)…The capital leans on a steep hill, bordering on a valley 6 or 7 li in length. On the north it is backed by high precipices.
To the north-east of the royal city there is a mountain, on the declivity of which is placed a stone figure of Buddha, erect, in height 140 or 150 feet. Its golden hues sparkle on every side, and its precious ornaments dazzle the eyes by their brightness.
To the east of this spot there is a convent, which was built by a former king of the country. To the east of the convent there is a standing figure of Sakya Buddha, made of metallic stone (teou-shih) in height 100 feet. It has been cast in different parts and joined together, and thus placed in a completed form as it stands.
To the east of the city 12 or 13 li there is a convent, in which there is a figure of Buddha lying in a sleeping position, as when he attained Nirvana. The figure is in length about 1000 feet or so. [Yuan Zhang (Samuel Beal, trans.)/Si-Yu-Ki pp 49-51
Now Yuan Zhang’s accounts are believed to be generally accurate, but when he mentions a 1000 foot reclining Buddha, perhaps indoors, you won’t be blamed for saying “but that’s incredible!”. Yet archaelogists, led by the indomitable Tarzis, not only believe that it’s there somewhere, but have been looking for it for several years. They have yet to find the big one, but the search throwing up many unexpected rewards—like a “new” 62-foot sleeping Buddha.
Citing Masson, in his translation of Yuan Zhang’s work, Samuel Beal mentions that there were five statues. The Taliban destroyed two. Archaeologists just found a third one. The elusive 1000-footer apart, there should be one more.