You can change the page size two ways. Right click on an empty area of the page or pasteboard area, select Page Options > Page Size and change the dimensions (you may need to select the Custom option first).

The other method is to drag the right or bottom edge of the page as needed.

Right click on the screen ruler (Ctrl L to display screen rulers) and select Grid and Ruler Options. I like to set the Major Spacing to 10 and the Number of Subdivisions to 50 or 100.

If you want to manually enter a dimension use px or pix, for example 250 pixels = 250px or 250pix. No space between the number and the units.