Adjusting your canvas height is a feature in Showit that allows you to optimize the overall length of mobile or desktop canvases according to the content included.
You have the option to either input a specific pixel height or to manually adjust a canvas height by adjusting the scroll bar located to the immediate left of the canvas window.
For this example we are going to expand the "About" canvas on the My Alive & Free template to show the extra text I have decided to include in my website. As you can see the text box and content clearly expands below the the lower edge of the canvas on both mobile and desktop:
If you select your canvas and navigate to the "Canvas" tab in the (right) properties panel you should see 2 mini tabs (Mobile and Desktop) that will allow you to set the canvas height to a predetermined height specific to the platform you may want to adjust. The other option to adjust the height is to locate the scroll bar on the current platform you may be editing and dragging the margin up or down:
(for larger image: Right Click>Open Image in New Tab)
For more tips on designing mobile and desktop views check out Designing Desktop and Mobile.