require 'rubygems'
require 'RMagick'
img = Magick::Image.read('myimage.jpg').first
width, height = 100, 100
thumb = img.resize(width, height)
thumb.write('mythumbnail.jpg')
scale_factor = 0.15
thumb = img.resize(scale_factor)
thumb.write("mythumbnail.jpg")
def thumb_no_bigger_than(img, width, height)
  img.change_geometry("#{width}x#{height}") do |cols, rows, img|
    img.resize(cols, rows)
  end
end
img.rows

img.columns

thumb = thumb_no_bigger_than(img, 100, 100)
thumb.rows

thumb.columns

thumb = img.scale(width, height)
thumb = img.scale(scale_factor)
thumb = img.sample(width, height)
thumb = img.sample(scale_factor)
thumb = img.thumbnail(width, height)
thumb = img.thumbnail(scale_factor)
thumb = img.crop(Magick::CenterGravity, 80, 100)

With an x, y offset relative to the upper-left corner:

thumb = img.crop(x, y, width, height)

With a GravityType and the x, y offset:

Error! (Exception?) Here's stdout:
NameError: undefined local variable or method `x' for main:Object
	from (irb):27
	from :0

thumb = img.crop(Magick::WestGravity, x, y, width, height)

With a GravityType:

Error! (Exception?) Here's stdout:
NameError: undefined local variable or method `x' for main:Object
	from (irb):28
	from :0

thumb = img.crop(Magick::EastGravity, width, height)