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