Stretch of the Milky Way

First we need to know about the reference frame against which we want to measure desired end of the Galaxy. Reference frame is a framework that is used for the observation and mathematical description of physical phenomena and the formulation of physical laws, usually consisting of an observer, a coordinate system, and a clock or clocks assigning times at positions with respect to the coordinate system.
Appropriate extension of such a reference frame needs to be bigger than the size of the galaxy itself, for the galaxy needs to be contained within it, but since we are well within the galaxy, that too, since our position in the galaxy is merely a point somewhere in it, so measuring desired edge of the galaxy in respect to our position would yield variable results, which is not desirable. So let us understand about the galaxy in respect to an imaginary reference frame whose extension is much bigger than the galaxy, as if you are seeing the galaxy from a distance so that you can have a complete look of the galaxy outside from a distance.
The Milky Way galaxy is about 100000 light years across and 100000 light years thick, which contains 100 - 400 billion stars including one of them is our Sun. The galaxy is so huge that you cannot spot our Sun in its complete map.

The galaxy is so big that it takes between 100 - 120 million years to complete one rotation around, whereas the Sun takes about 240 million years to complete one rotation in its orbit around the galaxy.