Identifying gauge of unmarked stranded wire
Wire gauge is based upon the conductor cross sectional area of the wire. There are tables all over the internet that show the characteristics of wire. Here is one for stranded wire.

For a given conductor cross sectional area the solid wire will have the smallest effective diameter. Stranded wire will have a slightly larger outside diameter due to the small spaces that exist between individual conductors. Different stranding (number and size of the individual conductors) will lead to different outside diameters because certain strandings will pack tighter together than others.