The Primitive Light takes the form of the object you ascribe it to. The light has no representation in OpenGL until attached to an object, whereupon it assumes a bounding box representation the size and shape of the original object used.
The light can simply emanate from geometry or the Sample Surface option can be used to take into account the geometry's surfacing. If you use sampled surfaces, the light color and intensity act as a multiplier for the object's luminous intensity and color surfacing
Mangalore warrior, resurfaced with PBSDF and a Primitive light collar
FFX Primitive light
When using FiberFX for Primitive lights, turning off Normalize is usually required
Roughly recreating luminous surfaces with spherical or area lights will be faster than using the primitive light