It varies between factions. I'm most familiar with my own, but I have experience with others, such as Nova Fleet Dynamics, Aethi Imperium and Vaygr.
Mass is a vague estimation. What makes up the mass is typically a more accurate way of determining what exactly a ship is - but in general, ships between ~10,000 to ~30,000 mass are considered frigates. Between ~30,000 and ~100,000, typically a destroyer, heavy frigate or the lowest echelons of cruisers. Between 100,000 and 200,000 most ships are referred to as various kinds of cruisers. Above that it varies wildly, but most consider anything above ~400,000 mass to be a titan or a rather large battleship/dreadnought.
Keep in mind, what exactly a ship is fitting can determine what it's called as much as its mass. Ships that focus more on raw offensive might are generally termed destroyers, while the all-arounders can be called frigates or cruisers. Titans are called titans solely due to how many a faction can reasonably deploy, so one faction's titan might not be a titan to another, and might even change to its own builders based on changes in manpower and economic ability.
StarMade has come leaps and bounds in the area of handling large ships. Nowadays, its typically the amount of entities and the relative power of each individual's computer. As a general rule though, I can't say I've ever seen more than approximately 2.5-3 million mass ever deployed in the same battle, with more average engagements coming in far lower, near the 1-1.5 million mass mark.
This may seem large, but keep in mind that once a faction is able to deploy large ships, they typically only deploy large ships.