Do galaxies change shape and size over time?
The size of a galaxy is dependent on how the matter was distributed in the early universe, and how it collapsed under its own gravity to form clumps of (dark) matter that the galaxies would form around.
The very earliest galaxies are irregular, but as matter fell towards them, and they developed a consistent orbital direction they formed into a disc. Gravity waves in the disc then show up as spiral arms. Large galaxies that are actively forming stars tend to have this structure. Small galaxies may never reach the point of forming a disc and remain irregular.
As galaxies collide and interact, the orbits of the stars are disrupted. The result of the merger of two large spiral galaxies is often a galaxy in which the stars orbit in all directions, which looks like an elliptical galaxy. Star formation in elliptical galaxies is often very low. Elliptical galaxies are the most evolved form of galaxies.
The time scale of this evolution is slow. It takes billions of years for galaxies to evolve from one form to another.
The details of the process of galaxy formation and evolution are still uncertain. The exact nature of dark matter will be important in understanding how galaxies form. The role (if any) that black holes play in galaxy formation is uncertain.
@James K -Good answer thanks - I would also like to know ,whether galaxies form around clumps of dark matter or on it.
Galaxies form *inside* clumps of dark matter. The matter can interact with itself, dissipate energy and collapse to form rotating discs of gas, from which stars form. (Smaller denser clumps probably form first and then combine to make the larger discs.) The dark matter remains as a roughly spherical halo. http://abyss.uoregon.edu/~js/ast123/lectures/lec25.html
Whoa, spiral arms are caused by gravity waves? Is there a good primer on how that works somewhere?
Try astronomy stack exchange ☺http://astronomy.stackexchange.com/questions/6232/questions-about-spiral-galaxy-arms