Many authors want to know how long a chapter or 
paragraph should be. Some ask when they should 
start a new paragraph.

Writers need to find a beat, much like music. Your 
writing should flow like music.

Of course a new chapter should start if the POV 
(point of view) is going to change, although this is 
not 100% of the time true, it is a good standard 
rule to follow for most writers. But there is no general 
rule on chapter length. A book may have chapters 
that are long and short. I've had chapters go on for 
over thirty pages with a four page chapter following it. 
A chapter should be as long as it takes for you to get 
that part of the story told. Some times a chapter will 
hold many scenes grouped together and at other 
times a chapter may only be one single scene. My 
novels change POV from chapter to chapter, which 
makes it easy for me to know when to start the next 
chapter. Like a beat in music I can feel when it is time 
to end the chapter and move on to the next one.

Long paragraphs are frowned upon, readers don't want 
to struggle through a page length paragraph. If you 
find this type of paragraph in your writing then you 
need to look closely at the suspect paragraph and see 
if there is a way you can break it up by adding dialogue 
or character thoughts. Keep the reader interested, that's 
the name of the game. When using dialogue always 
start a new paragraph when the dialogue changes from 
person to person. Don't have two people speaking within 
the same paragraph, this will confuse the reader and is 
not proper style.

The key to writing is finding your own beat, a beat your 
readers will also feel causing them to want to turn the 
page and keep reading, making it difficult for them to 
put your book down.