how to use rsizr
Choose the "rescale" tab to access this function. Resizing your image using rescaling, unlike retargeting, will treat each pixel of your image exactly the same. Rescaling works similarly to the resize functions of many image editing programs. To rescale, simply drag the handles on the sides of the image until the image is of the desired scale.
Hold down the Shift key on your keyboard while rescaling to maintain the proportions of your image:
You can also specify exact dimensions for the image by typing new pixel values into the Width and Height fields in the submenu:
Hit Enter on your keyboard to apply the changes. If you click the Maintain Proportions checkbox before you enter in a pixel value, the image will be rescaled by the original proportions of your image.

Starting over. If you want to undo your rescaling changes, click Reset.

Because the original image is modified when you rescale, if you go back to retarget mode, any seams you had previously calculated will be reset.
   opening pictures

   rsizr interface

   resizing pictures
    > rescaling

   saving pictures