Canvas size and image size??

[Martin]

Image size means the number of pixels in a unit area. when you resize a picture it merely spreads out or scrunches the image pixels. Pixels spread out image gets bigger.
On the other hand, if youre trying to make an image larger, and still keep your current dpi, well then PhotoShops got its work cut out for it. To make an image larger, PhotoShop must spread pixels apart. To keep resolution the same while stretching, PhotoShop must add new pixels to the image and decide what color they should be. When shrinking an image, PhotoShop must decide which pixels to remove.
PhotoShop is really good at adding and removing pixels, but you can generally only increase or decrease size by about 1/3 before images start looking yucky.

Then if you just need more space in the PhotoShop window, maybe youre trying to place two images next to each other, or add some text next to an image, then you have to go for canvas size.
Just like a painting is on a canvas, PhotoShop images are also on a canvas. If you need to add extra space in an image, youll just change the canvas size. The picture wont get any bigger or smaller, but there will be more space around it.