Later, you can change your mind and adjust the size of your page with the Canvas Size Option in the image menu.

Notice the anchor area. The filled in square represents your current page in relation to how it will be placed when the document is sized. You can pick any square.

You can also change your mind and adjust the size of your page and the image on it with the image Size Option in the image menu.

You can change the width and height in either the Pixel Dimensions area or in the Document size area. It also tells you what the files size will be next to Pixel Dimensions. You can use percentages, pixels, inches, etc.

Also note the "constrain proportions" box at the bottom. Click this to scale your image proportionately (without distortion).