Spoiler:
Brohan did this one, but it seems he deleted the blog so... guess I can do this quickly.

The feat

Spoiler:

Josef = 39 pixels

Boulder Diameter = 122 pixels

Boulder Diameter/Josef = 3.128

Josef = 1.7 meters

Boulder Diameter = 5.318 meters

Volume = (4/3)PIr^3

Boulder Volume = 78.709 m^3

Boulder Density = 2,700 kg/m^3

Boulder Mass = 212,514.3 kilograms or 212.514 tons

The boulder seems to be rolling at least twice as fast as the near beginning of the game cast can run away.

I could be more concrete guess, but not right now.

So, let's just use a placeholder of 24.8 m/s as the boulders speed (assuming the party, which at most by this point are around minor superhuman level, can leg it at peak human speed)

KE = 0.5mv^2

KE of Boulder = 65,352,397.536 joules or 15.6 kilograms of tnt

A bit feat given what they can do during the last fight, but this is a "rare" lifting strength like feat in FF.

Final Tally

Josef's Beginning of the Game Physical Strength = 15.6 kilograms of tnt