Each light is defined by a collection of properties - color, position, size, shape, opacity, etc. These properties are displayed in the Light Properties panel.
Top Tip: Double clicking on the text of a slider will reset the slider back to its default value!
Just select the light to edit the properties of that light. If the parameters are not visible, the light has been locked in the Light List and you will need to unlock the light before being able to edit it's properties. See Locking Lights. You can also quickly adjust the size, rotation and brightness of light using the Light Controls panel.