The glow of a lens flare is centered around the source object of the flare. The parameters on the Glow panel let you control each aspect of the glow.
Specifies the diameter of the glow of the lens flare as a percentage of the overall size of the frame. This value is separate from the overall size value set in the Flare Properties. This parameter can be animated.
Use radial, circular, transparency, and size gradients. Glow gradients are subtler than flare gradients, because they are glowing an area larger than a pixel.