I've just started writing my first proper novel, I've written short stories and started to write before, but never really had the motivation, but now I have a new, interesting storyline to write about, and I've just set the character and established some relationships, introduced the setting etc in the opening chapter but it only amounts to 2.1k words. Normally opening chapters, in my experience have been much longer. Anyone have anything to reply? Should I add more in, or does that risk cluttering and overwhelming the reader?
If to you the chapter seems good, and it seems to be fulfilling your purpose for it, I would not worry about it now.
If it is too short, what that really means one of the following: either you didn't establish as much in the opening chapter as you think you did, or else there's some other major problem (e.g. it's short because you're infodumping, or because you've forgotten to type in the vowels). In other words, "too short" might be a symptom, but it's certainly not a problem in and of itself.
If it doesn't establish enough, you'll find that out as you continue writing. You'll realize you're missing key information which needs to be established earlier, and at some point you'll rewrite to include that. That's fine.
If there's some other problem, you'll find that out when you pass your work around for feedback. Someone will tell you that a character is unclear, and you'll find yourself adding in huge scenes to establish him better. Someone will tell you the opening isn't very gripping, and you'll decide to add in a major sequence to the very beginning.
Or, readers will tell you that it's terrific and you shouldn't change a thing. Because if you know what you're doing, and your only concern about your opening chapter is that other books have longer chapters at their start - well, then that sounds like a pretty strong opening to me :)
Bottom line: keep writing. "Too short" isn't a problem. If there is a problem, you'll find it - later.
In my experience, chapter length does not matter. Your book may look more 'impressive' or 'official' with long chapters, but are they necessary to the book itself? No. As long as the first chapter does what the first chapter is supposed to do (be that introducing the protagonist, setting the scene, introducing the conflict, etc.), it doesn't matter if it is only 2.1k words. To use an example, if you read Harry Potter closely, you'll find that there are a few good stopping places in nearly every chapter. If the chapter ended at every one of them, the book would have been the same, just with shorter chapters. Length really doesn't contribute to content. Do keep in mind I am only an aspiring author myself, however.
In my experience, and from research into other's writing and professional opinions, the length or lack there of does not matter. Only you can know when your chapter is officially over. If you feel you have accomplished what you intended when you wrote that first chapter, than it is a success. If you feel it is lacking, than it probably is. Reread it and see what questions it leaves unanswered. Maybe allow a beta reader (someone to edit and proofread) to look it over and get an outside opinion on what it needs or does not need if you trust others to help.
My first chapter of my current book is 6.7k characters (11.5 pages at size 12 Times New Roman). And I had a similar question at first, except in the realm of it being too long perhaps. But I came to the same conclusion, that though it may be longer than some other books, it accomplishes what it needs too:
It sets the scene, Starts with trouble, Introduces the main POV and his environment.
Does your short first chapter have at least these three things? If so than I would leave it as it is for now and continue writing. Otherwise you can remain focused on this one chapter and never make it to the next chapter.
Hope this helps and does not ramble. :)
I'm personally of the opinion chapters exist more to organize information and events. It's functionally identical in most cases to have 20 chapters to having 10 chapters twice as long. Of course formatting or specific requirements might come in (publishers wanting specific chapter lengths, for example, and if this is a potential concern, you should research that specifically), but there's no solid rule and never will be for a "good" chapter length in numbers of pages or words.
I tend to end my chapters along thematic or time boundaries and I think such a practice makes far more sense than dividing chapters rather arbitrarily on page length like a lot of writers seem to do. I also like to open my chapters with quotes or such things, so keeping the chapters divided up along themes helps make the overall result look more coherent and planned. That said, if you think it's a sensible place to end the chapter, it probably is. You are, after all, the person who knows the story best, and should stand by your decisions in it.
That said, 2100 words for introducing the background seems a little small. It depends heavily on the setting, but that seems like it might be terse unless the setting is very narrow. The first chapter of the story I am currently writing is about 4000 words, ignoring the fact I have a prologue and some other formalities before that, and I consider myself incredibly to the point. I can't say without looking at the actual writing if there is a problem there, though. If you are bordering on an infodump first chapter, you might want to consider a prologue which simply does so, or ignoring it completely and add in flashbacks later on.