Adobe Photoshop cs4-What Does Canvas Size Really Mean?

Question

What are we actually looking at when we place a picture on a canvas?
For example, if I have a canvas of a width of 800 and a height of 1500 it appears on the screen to my eyes to be about 3 inches by 7 inches.
Then let's say I add the headline: Risk takers of all kinds think the same.
This headline will possibly take three rows to complete itself.
But then I can make the same canvas fit the screen.
Now the entire headline will fit on one row.
What confuses me is which image of the canvas is a "what-you-see-is-what-you-get" in terms of how it will actually look on my webpage?
In other words I guess my real question is how do I start with a canvas size that actually reflects my webpage or a defineable portion of it.
Thank You,
Jerry

Answer 1

You'll have to forgive me but at times I'm not really sure what I'm talking about or doing.
But an image or other content is built on layer canvas' I think?
It gets all confusing because you may or may not fill a canvas with an image?
Assuming you do fill a canvas with an image then I think the pixel dimensions and resolution will be directly tied to the canvas size?
But I don't think any of that is my real point.
I'll try to speak to my confusion using words other than those I first used.
For example let's say you are making a squeeze page for your website.
So you start with a headline that reads as follows: INTERNET MARKETER STICKS TO HIS GUM!
Let's say those words are in large print.
On my computer and I am using a small screen, if I create a canvas of a width =800 and height=1500 it occupies a space of
of about 3" by 7". 
Now (and this part is now becoming like what I wrote above) those six words in large letters will occupy three to four rows on my computer.
So in my mind I am thinking that when I eventually publish that page that the headline should occupy four lines. Make sense?
But if I click the "fit to screen" Photoshop button, the canvas becomes larger on my screen and the entire headline will now fit on a "single" line.
So now in my mind I am thinking that when I eventually publish that page that the headline should occupy one line.  See my problem?
I guess my point is am I looking at WYSIWYG in the first, second or neither case?
In other words what is the meaning of what I see when I look at a canvas?
Thank you for your help.
Here's one more attempt.
Let's say you spend 12 hours designing a canvas in Photoshop that you thought would properly fill your webpage, and then you discover to your dismay that instead of your headline fitting properly only 37% of it does.  This would make you very unhappy and perhaps others.
So what I'm trying to understand as a beginner is how does what I see on a canvas in Photoshop cs4 relate to a webpage.