Two excellent questions.
No, galaxies have different sizes and structures, actually.

The largest and most massive galaxies tend to be giant ellipticals, those believed to have been created by the merger of other galaxies. The most massive known galaxy -so far- is ESO 146-IG 005, a supermassive galaxy within the galactic cluster of Abell 3827, about 1.4 billion light years away.

[Image credit: Gemini Legacy Image: R. Carrasco et al., Gemini Observatory/AURA]

This immensely large galaxy is estimated to be as massive as 30 trillion suns! For comparison, the material comprising our home galaxy, the Milky Way, contains a comparatively meager four billion solar masses. In the image above, captured by the Gemini south telescope in Chile, we can see the nuclei of four galaxies which ESO 146-IG 005 is currently ‘devouring.’ Remarkably, this mammoth galaxy will only grow more massive as it cannibalizes galaxies within its proximity. As it consumes more material and consequently becomes more gravitationally powerful, ESO 146-IG 005 will be able to incorporate many more galaxies into itself: a perfect example of the ‘snowball effect.’

The smallest known galaxies are known as ultracompact dwarfs. The least massive ultracompact dwarf yet discovered is M60-UCD1, a tiny galaxy consisting of approximately 140 million stars within a diameter of 300 light years. (1/500th the diameter of the Milky Way.)* This ultracompact dwarf galaxy, 49 million light years away within the Virgo Supercluster, is likely the striped core of a massive galaxy that lost much of its material when it encountered the galaxy M60 about 10 billion years away. Notice that its name, M60-UCD1refers to its proximity to M60 and its size classification.

An image of M60-UCD1 as captured by the Hubble Space Telescope

Note: In this answer I am referring to those galaxies which contain a supermassive black hole in their nuclei.       There are even smaller systems, such as dwarf spheroidal galaxies such as the Fornax Dwarf and NGC 147.   However, I didn't consider this class because of the lack of the SMBH at their centers.

As for the second question: the Andromeda and Milky Way galaxies are destined to “collide” within 4 - 6 billion years because the two galaxies are relatively close to each other: 2.537 million light years. One can consider this separation to be slight when viewed on the cosmological scale. The mutual gravitational attraction is drawing them together at about 500,000 miles an hour. (That velocity will increase as they draw closer together.) One can watch a brilliant simulation of this merger here

Also, the Andromeda Galaxy is actually larger and more massive than the Milky Way. The Milky Way’s diameter is 100,000 -180,000 light years (still uncertain), the diameter of Andromeda is nearly 220,000. One can see a scale model depiction of both galaxies and their separation distance below:

[Image credit: Jan van der Crabben - Own work, CC BY-SA 4.0, File:Milky Way and Andromeda in space, to scale.jpg - Wikimedia Commons

I hope this answer proves helpful.

*Were you to stand on the surface of a planet within this galaxy you’d see about one million stars throughout the night. Compare this value  to the 10,000 stars visible throughout the night on Earth. And, yes, the probability of life within this galaxy is reduced due to its density.