So, when do you know when a class is too big? The single responsiblity principle nails it, however to me there seems to be a fuzzy line. The single responsibilty principle says every class should have one responsiblity. In my pong example, I had a GameLogic and GameDraw class to seperate these componets of my program. But imagine this: If my paddle stores it's own sprite, why don't I just have the Paddle draw my Sprite? Surely that won't interfere with handling the GameLogic!
However I kept the single responsibility principle? Why? Because without that line between what's doing what, my code would become extremely confusing. And like I said in my pong example, I want to make sure a change in my GameLogic doesn't mess Up GameDraw. Well, since the Paddle class' responsibility is to handle it's own logic, that means that making a change to my Paddle class' logic can affect how I draw the paddle. And even though I know I probably won't change the Paddle class, it's best to make sure that no matter what I'm keeping my code clean and expansive.
So back to the topic of seperation. There should be seperation between classes, since they should have one responsiblity. When you start getting into more detailed programming, we have to bring in composition. What's composition? Composition is idea that a class is "made-of" rather than "is a". However a class can be both.
If there's ever a blurry line for you, try thinking about what your class is made of and if anything in it should be it's own object. Likewise, if my Paddle Had four floats instead of a normal Rectange, I should create a new class called rectangle that my class is "made-of". My Paddle class is made of an image, a sprite, a position (Vector2f if we want to get more detailed ), and not much more. My Paddle isn't "made-of" a ball, so when I handle collision detection in my paddle I pass in a ball to the CheckCollision() function. You should always make sure the objects in your class make up the class, rather than the objects in your class being what the class uses.
I hope you enjoyed this, and if you don't know what my pong example is, you can find it here: Pong Example!