There is no rigid rule that applies to the size of the main map in relation to the overview map. It depends on what you want the map to communicate and who is the audience. How are you going to use the map and is it a wall map, book map or brochure?
Cartography is the discipline dealing with the art, science and technology of making and using maps (Definitions at the bottom of the page). The science part deals with scales, projections, geometry etc.
The art part, which relates to your question, deals with proportion (this is your question), harmony, contrast etc. And in turn these can be driven by color, line thickness, tone, use of space etc all subjective parameters. However, there are basic guidelines that, when followed in the course of map design, will result in a map that delivers on its purpose.