Inkscape

Crop Image
http://goinkscape.com/how-to-crop-an-image-in-inkscape/


 * 1) Rectangle tool and draw out an area you’d like to keep
 * 2) Use the Selection tool to select the image and rectangle
 * 3) then head up to Object > Clip > Set.

Set Ruler Units
File Menu -> Document Properties -> Default Units

Scale Image
Select -> Object -> Transform

72 pixels per inch common

Resize Page to Image
Edit -> Resize Page to Selection

(or Ctrl + Shift + R)

Zoom
Ctrl + Mouse Wheel

Scroll
Hold Center Mouse Button + Move Mouse