The Lens size really depicts everything to do with camera coverage, the smaller the lens size the more width of view you obtian. The following chart give a coarse overview of how this affects the FoV (Field of View).
This is only a coarse overview as each camera make is different depending on the chipsets and configuration that the manufacturers utilise.
A fixed lens size is a predefined lens that gives you a fixed FoV (Field of View)
Varifocal lenses, whilst more expensive, give you the option of changing the lens sizes once the camera is in its position, manually increasing or decreasing the FoV.