Answer
About 3 feet across.
Work Step by Step
The volume of a sphere is $V=\frac{4 \pi R^3}{3}$.
Solve for the radius $R=(\frac{3V}{4 \pi})^{1/3}$.
For a 1-ton rock, first find the volume.
$$V=(1T)(\frac{2000lb}{1T}\frac{1ft^3}{186lb})=10.8ft^3$$
$$d=2R=2(\frac{3V}{4 \pi})^{1/3}=2(\frac{3(10.8ft^3)}{4 \pi})^{1/3}= 2.74 ft$$
