To use a Sketch Picture effectively it would be helpful to scale it to the correct size.
(Tools Menu > Sketch Tools > Sketch Picture)
To do this:
-Generate some sketch geometry such as a line, circle, or rectangle to establish scale for your part. (i.e. hole centers, length of an edge or size of main feature)
-Dimension the geometry to the required size.
-Insert the Sketch Picture and scale/drag the picture to match the sketch geometry.
The other option would be to crop the Sketch Picture in an image editing program to the edges of the geometry. When inserting the Sketch Picture you can define the size of the image for the geometry.