That's insulation diameter. For every wire size there are many insulation types and thicknesses.
Inside the car, general purpose primary wire insulation is fine.
Often I can identify wire gage of original wires by relative size because I am familiar with them. But I still check the book. New wire insulation may or may not be thicker depending on the material and manufactor. I don't eyeball estimate that.
Its possible to convert from metric wire diameter to wire gage but its probably best to simply look it up in the books.
For example, what gage is the yellow S2 wire in the diagram I posted above?
Need to know the car and options to even hope to be more specific about what you have there.
That looks like it could be a twin lock connector as shown here Sources for Chrysler type wire terminals