Using Firefox, you can enlarge an entire web page by simply pressing CTRL +. What this does is proportionally enlarge the entire web page (fonts, images, etc).
How can I replicate the same functionality using simply CSS?
Is there something like page-size: 150% (which would increase the entire page portions by x%?)
If your CSS is constructed completely around ex or em units, then this might be possible and feasible. You'd just need to declare font-size: 150% in your style for body or html. This should cause every other lengths to scale proportionally. You can't scale images this way, though, unless they get a style too.
But that's a very big if on most sites, anyway.
You might be able to use the CSS Zoom property - supported in IE 5.5+, Opera, and Safari 4, and Chrome (verifed, please check before downvoting).
Firefox is the only major browser that does not support Zoom (bugzilla item here) but you could use the "proprietary" -moz-transform property in Firefox 3.5.
So you could use:
div.zoomed { zoom: 3; -moz-transform: scale(3); }
As Johannes says -- not enough rep to comment directly on his answer -- you can indeed do this as long as all elements' "dimensions are specified as a multiple of the font's size. Meaning, everything where you used %, em or ex units". Although I think % are based on containing element, not font-size.
And you wouldn't normally use these relative units for images, given they are composed of pixels, but there's a trick which makes this a lot more practical.
If you define body{font-size: 62.5%}; then 1em will be equivalent to 10px. As far as I know this works across all main browsers.
Then you can specify your (e.g.) 100px square images with width: 10em; height: 10em; and assuming Firefox's scaling is set to default, the images will be their natural size.
Make body{font-size: 125%}; and everything - including images - wil be double original size.
I think its impossible to do that purely with CSS - you may need you get your hand dirty with Javascript to make sure the job gets done properly.
Jon Tan has done this with his site - http://jontangerine.com/ Everything including images has been declared in ems. Everything. This is how the desired effect is achieved. Text zoom and screen zoom yield almost the exact same result.
CSS will not be able to zoom on demand, but if you couple CSS with JS, you could change some values to make a page look bigger. However, as it has been said, this feature is standard nowadays in modern browsers: no need to replicate it. As a matter of fact, replicating it will slow down your website (more things to load, more JS or CSS to parse or execute and apply, etc.)