What will the mass of the new galaxy be? When the Milky Way galaxy collides with the Andromeda galaxy, what will the mass and volume of the new galaxy be?
The mass will be slightly less than the combine masses of the 2 galaxies since some of the stars will be hurled away. 

Since the disks of the galaxies are at an angle to each other, the volume would be (roughly) the volume of the 2 galaxies as they collide. Eventually, the volume will decrease (I'm guessing to between 60 to 70%) as the stars adapt to their new environment and the central massive black holes combine.