By 64 times larger, I would expect you mean 64 time more massive, or 4 times longer, wider, and taller.

An object may be moved by rolling or sliding.

If it isn't perfectly round, it will tend to lie in an orientation where its center of gravity is as low as possible. Moving water will tip it over onto another side. If it is 4 times larger, the torque needed will be 64 times larger because of the bigger mass, with another factor of 4 because the center of mass is 4 times farther from the edge. So the torque must be 256 times bigger.

The source of the force is diverting water from a straight path for larger Reynolds numbers (larger objects, faster flow, lower viscosity) or surface friction for low Reynolds numbers (smaller objects, slower flow, more viscosity). A stream rolling a rock would likely flow fast enough to be turbulent, which is a sign of high Reynolds number.

A rock 4 times larger has a cross section 16 times larger, and diverts that much more water. For high Reynolds number, flowing water exerts a force proportional to $v^3$, so doubling the water speed increases the force by a factor of 8. So the force is 128 times bigger. The center of the cross sectional area is 4 times taller, so the torque is 512 times bigger. You could roll a slightly bigger rock than your source says.

Friction is harder to figure out. This is not a simple case of sliding friction. Water can be a lubricant, and flowing water can lift a rock or press it down harder. But ignoring that, we can say that the contact area grows by a factor of 16 and an weight by a factor of 64. The force of friction would be 1024 times bigger. So doubling the water speed would not move a rock 4 times bigger.

By 64 times larger, I would expect you mean 64 time more massive, or 4 times longer, wider, and taller.

An object may be moved by rolling or sliding.

If it isn't perfectly round, it will tend to lie in an orientation where its center of gravity is as low as possible. Moving water will tip it over onto another side. If it is 4 times larger, the torque needed will be 64 times larger because of the bigger mass, with another factor of 4 because the center of mass is 4 times farther from the edge. So the torque must be 256 times bigger.

The source of the force is diverting water from a straight path for larger Reynolds numbers (larger objects, faster flow, lower viscosity) or surface friction for low Reynolds numbers (smaller objects, slower flow, more viscosity). A stream rolling a rock would likely flow fast enough to be turbulent, which is a sign of high Reynolds number.

A rock 4 times larger has a cross section 16 times larger, and diverts that much more water. For high Reynolds number, flowing water exerts a force proportional to $v^3$, so doubling the water speed increases the force by a factor of 8. So the force is 128 times bigger. The center of the cross sectional area is 4 times taller, so the torque is 512 times bigger. You could roll a slightly bigger rock than your source says.