My steampunk series of posts has so far focused on what's possible with steam power. However, there are also some limits to the capabilities of steam technology, primarily focused on the needed parts for steam machines to operate and the size of the related engine.
Steam-fueled machinery, unlike contemporary electronics, requires moving parts in order to operate. This means gears, flywheels, pistols, belts, and various sundry items to maintain mechanical motion. If you've ever looked under your car's hood while it's running, you can see the type of machinery I'm talking about.
Because steam machines requires this, the mechanism is going to be more complex and, depending on your time period, larger than what we see now. Take a look at this steam car prototype from the 1770s vs the 1924 Doble or the Stanley Steamer. The last two are much more advanced, but you can see from this diagram (sorry it's in German), the parts that a steam car needs to have in order to run. And here's some diagrams of how a steam engine works.
For any other steam technology, whether it's an airship, tank, or battleship, some things to consider are boiler and engine size, water supply, and fuel storage. Sure you can have a massive steam engine powering your humongous aerial battleship, but you're going to need a boiler that's a proportionate size to create all the steam needed to make the airship rise. That's going to necessitate a room at least the size of a standard battleship engine room (roughly 60 feet long by 27 feet wide). The USS Texas had two engine rooms that size, as one example.
What's this mean for your naval/aerial vessels? They need to be comparable sizes to vessels that actually ran on steam power. And if you intend to have the airship be a zeppelin, well then you have another issue with the balloon size. Note how much larger the balloon is than the gondola where the people sit. This should probably increase proportionately with each addition to the underside. Or you could always go the Girl Genius route (Castle Wulfenbach) and make the balloon itself be part of the airship. Your choice really.
In Part 2, we'll talk about the other limits I mentioned.
Note: Involve magic instead of straight technology, and many of these limits go out the proverbial window.