The three sizes are not as different as you may think, but it does impact the game. In addition to the major races, a wide variety of races that were encountered in the television show are included, such as the Mintakans or Bentic. These races control a single planet but otherwise function much like a major race. You can conquer them, make alliances with them, and so forth. You can choose to play without them if you want, or add more or less based on a pre-game preference.
Game play is similar in many respects to other examples of the breed.
You control your ships to move around the galaxy, direct research and construction of your systems, negotiate treaties and alliances with other races and wage war either directly or through espionage. Research breakthroughs will generally yield either a new type of building you can construct, and upgrade to an existing basic structure or upgrade your weapons or hull types. The upgrades to structures seem like a neat idea at first, but in practice they make for a lot of tedium. Upgrades allow each worker assigned to a structure to produce more food, energy, manufacturing, science or espionage each turn. But you don't upgrade for free.