OK this can get confusing at times so hopefully this makes sense.
On it's own measurements like you stated (e.g. 11"x17") don't really mean anything without an accompanying resolution.
When you create a canvas in Photoshop using paper measurements in inches, you also set a number in the pixel resolution as well. Photoshop then takes those figures and works out how many pixels are needed to create a canvas so many inches wide/high x how many pixels in each inch. e.g. If you create a canvas 11" x 17" @ 300ppi, Photoshop will give you a canvas 3300 pixels wide x 5100 pixels high (300 pixels for every inch you told it you wanted).
You could print a 11" x 17" canvas that was physically smaller (uses fewer pixels to create it) by defining a smaller figure in the resolution box at the beginning. e.g. a canvas 11" x 17" @ 72ppi would give you only 792 pixels wide x 1224 pixels high. This would then print to the same size on paper, but each pixel would appear larger to fill the same space.
The fewer pixels in the second canvas above would give you less potential detail when painting and a lower quality print, but it would also use less memory because fewer pixels are in use. So if your computer can't handle the extra pixels created in a higher resolution, then you need to trade off slightly in print quality and create your canvas with a lower resolution. The smaller image (in pixel terms) will print the exact same size as the larger one but would give a slightly fuzzier print.
This is why people can paint images printed big enough to fill a billboard without needing the most powerful comuter known to man. The pixel resolution is lower therefore making each pixel fill a much bigger area when printed.
