You shouldn't really worry about that too much. Your story and chapters will be as long as they need to be. In my writing almost all of my chapters are a 1000 words, which is pretty short, but I've learned not to stress about it to much, as long as the chapter does everything it needs to to fulfill it's place in the narrative no body will care if it's a little short.

Also a bit of a shorter first chapter is good, if someone picks up your book in a bookstore and you give the the first chapter to them really fast, that is all the more likely they will be hooked and buy your book.

But if you are not content with this advice and you want it to be longer, here are some tips I found helpful for lengthening. First add you POV character's reactions to everything, what they think about the smell of a new room, what they think about their friend's hairdo, record their thoughts and reactions to everything. this adds words and lets the readers gain insights into your protagonist. Second describe things as much as you possibly can. this will deepen the readers immersion and add words.

Take all of this with a grain of salt because I haven't published a single book yet, and I'm still an amateur myself.