It depends on the information you have.
You could put the sphere on a flat surface and lower a horizontal plane onto it so that it just touches the top of the sphere. The distance between the flat surface and the horizontal plane is the sphere's diameter; the radius is half that.
Or you could measure its volume by measuring the amount of fluid (water) that it displaces in a measuring container or the overflow from any full container. Then use the formula V = 4/3*pi*R3 to work out the radius.
If you knew the density of the material of the sphere, you could measure its mass and work out its volume that way.
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The radius of a sphere is 1/2 of its height.
Treat the 3D sphere as a 2D circle. The radius for the sphere is the same radius as for the circle. No matter where on the sphere you place a mark, the distance (radius) from the mark to the centre of the sphere will always be the same as the circle.
The surface area of the sphere with the radius doubled is 200 units2.---> Confirmed
It's the extension of the sphere's radius drawn to that point.
360 units cubed