How to Resize the Easy Way
In order to resize your images, you need to first determine how you'd like for them to be resized. The options that sips gives you are resizing to a specified width, specified height, or specified size. Alternatively, you can resample your images so that either the width or the height (depending on the dimensions) will be a specified amount, and the other dimension will be resized proportionately. We'll take a look at which command to use below:
If you want the image(s) to have a specific width: Use the command "sips --resampleWidth [number] [filename(s)]", where [number] is the width, in pixels, you'd like the image to be, and [filename(s)] is the name of your file, a space separated list of files, or *.* to signify all files in the directory.
If you'd like for the image(s) to be uniform height: Use the command "sips --resampleHeight [number] [filename(s)]", where the variables are as described above.
If you want to resample the image(s) to be an exact height and width: Use the command "sips --resampleHeightWidth [number] [filename(s)]".
If you want the image(s) to be resized such that they fit in a bounding box: Use the command "sips --resampleHeightWidthMax [number] [filename(s)]". This command will take the longest dimension, shrink or expand it to [number], and shrink or expand the other dimension proportionately. Thus, if you have an image that's 400x200 and set the number to 100, the resulting image will be 100x50. Conversely, if the image is 200x400, the image will be 50x100.
When you've decided how you'd like to resize the images, simply enter the above command by opening Utilities -> Terminal and typing the command there. If you'd like to resize all the images in a directory, enter "sips --resample[mode] [number] /[pathtomydirectory]/*.*".