I would personally define what a chapter is first.
To me, a chapter is a block of the story that can stand alone within the context of the whole narrative. So if you wrote 350 words, then all those words have to build up to what you want to tell the reader in that single block. One way I do to help me with this is plan out the story in bullet points. Say I want to make a story of someone trying to compete in a bicycle race but they are not fit physically to do so. I would outline it like this:
- Person A has had a desire to join a bike race for so long but he's not fit. He's too poor to afford a bicycle that can compete.
- He sees someone about to be run over by a car and rescues him. However, the victim gets his leg injured.
- Person B is thankful to Person A for saving his life but he has injured his leg and can't compete in the upcoming bicycle match 6 months from now.
- Person A confines in him that he wishes he could do so and take his place. Person B declares that Person A can do so and is willing to train him.
- Person B trains Person A for months and Person A is now fit before the race starts.
- On the day of the race, Person B gifts him his prized bicycle and wishes him good luck. Person A is more determined than ever.
- Race day comes and Person A goes for it. He does well in the beginning.
- Half-way, he's feeling the pressure as the competition starts getting fierce. He soon realizes that he pushed himself too much in the beginning.
- He's on the last leg of the race and he feels he can't do it. He sees his coach up ahead and this gives him the courage to try.
- He tries his best and he manages to get 3rd place.
- The coach congratulates him but he feels so bummed out he didn't win. The coach assures him that for his first try, he did better than expected. The coach tells him that next year he will surely win.
- Person A feels better and determines to be better next year.
This is a small example. If it was going to be an actual story, I would expand it with a bit more bullet points. The gist of it is that the important parts of each passage are summed up in as few sentence
s as possible
. Each chapter can be a single bullet point or you can take two, or even three, but pretty much what you planned to do with each chapter must be able to be contracted into a single line and then expanded upon. If this is too hard, you can then make bullet points within the bullet points, such as the first example:
- Person A has had a desire to join a bike race for so long but he's not fit. He's too poor to afford a bicycle that can compete.
- Person A has always admired bike racing since he was little.
- He's had health problems that kept him from taking up the sport.
- Even though he went to college, he stuck on a dead end job with student loan debt, so he can't afford a bike that can actually compete.
- He passes by a bike shop every day and stares at a bike he's always wanted to buy.
And if that isn't enough, you can just keep making bullet points within bullet points until you believe you have enough to fill up a page with whatever information you believe the reader will need for that chapter.
So for your original query about word length, if you are truly worried about the word numbers, then this is a good way of figuring out how to fill up an entire chapter with a desired word amount that tells all the information the reader needs in that chapter in a satisfactory way.
This is my personal method. I'm sure others have their own, but I'd figured it can give you some ideas of your own.