The average length of chapter of a real book is, depending on genre, somewhere between 3000 to 5000 words.

Wattpad seems to have one as well. If a chapter is too short, readers get annoyed with with the interruptions that come from chapter changes, example being advertisements. Not too mention, in short chapters, nothing can really happen. And if something happens, it can't be very detailed.

Fanfiction by nature is serialised. To keep people reading, when they have to wait for a new chapter, each chapter needs to be entertaining. A chapter needs to progress the story in some way. If not, it feels like useless filler and a waste of time. The chapter needs to end on a note, maybe a Cliffhanger. But not every chapter. Don't end every chapter on a Cliffhanger.

To fill a chapter with worthwhile progression, a chapter needs space.

But!

Wattpad's "attention span", for a lack of a better word. I noted that long chapters may push readers away on here. Which means a 5000 word chapter may be a bit too long.

I wrote a lot of 1000 word chapters. A good length, though it felt too short sometimes. So I would say 2000 words is better. Anything over 3.500 may be a bit too long.

But whatever length you choose, keep the chapter length consistent.
Keep the length around 1.200 for each chapter. Don't jump from 1000 to 2.500 to 500 to 1.500 etc. Keep it consistent.

Though, Prologue and Epilogues can be longer or shorter. Last chapters can be longer as well.

Not shorter than average. It would be disappointing if the finale is shorter than the average chapter.

Last but not least, if a One Shot is longer than 10.000, maybe it is better if that one gets turned into a small Fanfiction instead.