The canvas can be adjusted by both width and height in the Logo Builder and Graphics Builder and only by height in the Header Builder. The canvas in the frame builder works differently and is covered in detail in the Frame Builder Tutorial.
To adjust the size of the canvas (background), just click the "Background" button at the top of the left navigation menu.
Once you click the "Background" button, the "Background Size" and the "Background Color" tool palettes will open.
By using the Background Size tool, you can quickly adjust the canvas by simply entering in the width and height in pixels and clicking the Enter key. Or alternatively, you can simply use the Width and Height sliders, or the use the small arrow keys for incremental changes.
To change the background color of the canvas, just use the Background Color tool. You can either use a transparent background or colored by checking one of the two selections.
Please note that the background color in the Logo Builder is for preview purposes and will never be included in the finished image. Any images produced by the Graphics Tool will always include the background.
If "Use Colored Background" is selected, you can either use a solid color by using the color picker, or use a gradient by using the gradient tool. The app will simply use whichever choice you use last.