Here is the MIT explanation of speed, from
Ball
A round ‘sprite’ that can be placed on a Canvas , where it can react to touches and drags, interact with other sprites ( ImageSprite s and other Ball s) and the edge of the Canvas , and move according to its property values.
For example, to have a Ball move 4 pixels toward the top of a Canvas every 500 milliseconds (half second), you would set the Speed property to 4 [pixels], the Interval property to 500 [milliseconds], the Heading property to 90 [degrees], and the Enabled property to true . These and its other properties can be changed at any time.
The difference between a Ball and an ImageSprite is that the latter can get its appearance from an image file, while a Ball ’s appearance can only be changed by varying its PaintColor and Radius properties.
Canvas.Height and Width are in pixels, and are available after the Canvas appears on the screen, so those should help you fit to your devices.
Here are some animation projects you can try on different devices. (I avoid images and their headaches.)
Angry Balls V3 Documentation - Google Docs