You can adjust the size of the Falloff Radius in a number of ways.
To adjust the Falloff Radius relative to its current size, press b while dragging the mouse left or right. The cursor changes to a circle representing the size of the falloff area starting at the current Falloff Radius. Relative resize is useful when you are making multiple small adjustments to your model.
To adjust the Falloff Radius starting from an absolute size of 0, press b + left or right. The cursor changes to a circle representing the size of the falloff area starting at 0. Absolute resize is useful when you are first starting to make Soft Selection transformations or when you have lost track of the size of the Falloff Radius.
You can also adjust the Falloff Radius in the Tool Settings editor.