How to change width and height of canvas/artboard in Illustrator
To change width and height of an artboard in Illustrator to a specific valueGo to Window->Artboards and click on "Artboard" option, or make sure it already a check mark next to it. Locate the small Artboards sidebar that will be added to the Layers panel, or free floating on the screen. Single-click on the small icon to the right of entry "Artboard 1" to open the "Artboard Options" dialog. Type width and height of your new document view area using points (pt) values.
It appears that Illustrator does not allow pixel (px) values to be used for sizing the document canvas area.
Here is the visual process of changing width and height of the artboard.
Step 1: Find the Window->Artboards option, and make sure it has a check box next to it. If not, click on it to enable the Artboards tab.
Step 2: Now that the Artboards tab appears within the Layers settings box (or it may be floating freely in another area of your screen, or paired with another settings window) click on the small icon next to the artboard you want to change width and height for. It looks like this:
Step 3: Type in the new values for width and height parameters of the artboard. Note that it appears there isn't an option to specify width and height using pixels.
Click OK to apply changes.