The percentage located at the top of your screen shows you your scale.
You can scale your sketch in and out by holding the circle button and using a pinching gesture with two fingers.
You will only need to hold the circle button if you aren't in Pen mode. If in the Pen mode you can use the fingers gesture only.
You can also rescale your view by clicking on this percentage and choosing any of the options.
A. 100%: This will resize your view back to 100%
B. Arrows facing the corners of a box: This will resize your view to see your whole sketch
C. Home button: This will reset your drawing plane and view to what you had before you started your sketch when you first entered the new sketch (the drawing plane should go back to the origin axis)