Inkscape
Crop Image
http://goinkscape.com/how-to-crop-an-image-in-inkscape/
- Rectangle tool and draw out an area you’d like to keep
- Use the Selection tool to select the image and rectangle
- 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