Q: What balances the force of gravitation and prevents the Sun and other stars from collapsing to the center of the Milky Way galaxy?
A: Centrifugal force. (230 million years per revolution)

The relationship between the size of the super massive black hole at the center of the galaxy and the size of the galaxy seems to be related. The implication is that the speed of the matter circling the SMBH eventually increases to a point where matter quits falling into the hole.

http://oposite.stsci.edu/pubinfo/PR/97/01/PR.html