There was no option to adjust the edge_width like there is in the preferences for vertex_size and face_dot_size.
I only added the option for 3DView and UV/Image Editor, and limited both to a max size of 5 pixel, since the edges do not look very nice with too high values.
In the UV Editor only, there are always black outlines on the edges, I could not find a way to reduce the increasing thickness of these black outlines.
The default edge_width of 1 pixel.