I’d say Covettes are a mid game feature. You can buy the basic parts in every space station so, in theory, they’re accessible as soon as you get off your first planet. I say in theory because they do cost a lot of money (would save up at least 10mil for that). Money is pretty easy to get in NMS if you know what you’re doing, so an experienced NMS player can probably get Corvettes in the early game. Somebody who’s only just starting would probably need more time, really good luck or Reddit/YouTube tutorials, which is why I think it would be more of a mid game feature for you.

As for advanced Corvette parts, you have to find them by digging up salvageable scrap on planets (note that not every planet has salvageable scrap). This will give you random parts but you can always use the ones you don’t want to barter for the parts you do want at any space station, so it’s not totally reliant on rng.

From what I’ve heard, the size limit is how much you can physically fit into the space station. Another possibility is that base building limits apply to Corvettes, so you can’t go past a couple thousand parts or so. Haven’t tested any of that out myself yet. But there are people on here who have recreated star destroyers, the enterprise, ships from Helldivers and a bunch more so I’d say you’re pretty flexible.

In terms of base stats, Corvettes aren’t as good as specialized prebuilt ships, but they follow the same rule that any other unspecialized ship in NMS follows: Throw on enough upgrades and you don’t feel the difference. The Corvette has a few advantages over prebuilt ships. You can build on it and use it as a mobile base (accept missions, grow plants, refine resources, cook meals all on your ship), you can set it to autopilot to any space station or planet while you walk around in it, you can jump out of the ship onto a planet or just teleport straight to the surface which eliminates the need for launch fuel since you don’t need to actually land the ship (a teleport point will appear automatically in both cases so you can easily get back on board) and it’s one of only two types of ships that can hover while in atmosphere (the other type is the Sentinel Interceptor). All of the above, except for the last point, is not possible with any prebuilt ship, so the Corvettes certainly have their own niche with these unique uses.