ImageMagick size cheatsheet
To use with Carrierwave, Paperclip, Dragonfly and other gems that use minimagick gem.
'400x300' # resize, maintain aspect ratio
'400x300!' # force resize, don't maintain aspect ratio
'400x' # resize width, maintain aspect ratio
'x300' # resize height, maintain aspect ratio
'400x300<' # resize only if the image is smaller than this
'400x300>' # resize only if the image is larger than this
'50x50%' # resize width and height to 50%
'400x300^' # resize width, height to minimum 400,300, maintain aspect ratio
'2000@' # resize so max area in pixels is 2000
'400x300#' # resize, crop if necessary to maintain aspect ratio (centre gravity)
'400x300#ne' # as above, north-east gravity
'400x300se' # crop, with south-east gravity
'400x300+50+100' # crop from the point 50,100 with width, height 400,300