I find it boils down to a number of things.
The first and foremost is the old notion of a chapter beginning at the beginning of a scene and ending at the end of a scene.
However this is more a loose guideline than a rule and the exceptions crop up quickly when you start hammering out the scene.
When dealing with close third and more specifically first person--it can be tiring to try to cover the scene continuously, because there are just some things within the character's thoughts and actions that begin to grind the story to dust if you try to follow them throughout the entire narrative. That is to say that you will find yourself jumping forward from one active scene to the next because the whole plot is not some rollercoaster ride from front page to last page(well, not always)i.e.; you don't need to follow them into the bathroom or loo and to their meals and sleep time or sometimes not even the ride to get to the other active scene[although I seem to recall a Sinclair Lewis novel where he used the travel time to do backstory while his character reflected on recent events and taking himself into past events.] If you started making all your scene breaks into new chapters you'd likely end up with hundreds of small chapters.
You could look at a chapter as something that begins a specific action or set of actions that will come to some conclusion--of some sort--by the end of the chapter. Then you might or might not break into several scenes to get there so that the series of scene breaks make up their own contiguous scene. This leads to some sort of conclusion and then the beginning of a new chapter. However I've seen plenty of novels where the conclusion ends up being a cliffhanger of sorts.
No matter how it works out; one thing I try to do is begin the next chapter the same as I begin the novel. Starting with something that is framed in a way that will draw the reader into the story.
Then in the case of multiple scenes that make one chapter; start each scene as though the reader just took a quick break, coffee, ice-crème, or some such, and you need to get them invested into the story again.
In short, a chapter should be short enough that the reader doesn't wet their selves trying to finish it and yet long enough they don't start thinking about wandering around the house.
The real answer though, is, a chapter is as long as it needs to be and as many scenes as it takes to get it there.
