Select your crop tool, then enter the width and/or height of your final version. Enter px for pixels. In this example a width of 194 px and a height of 205 px is entered. Leave the resolution blank.
If you would like to resize your photo for a medium size on Flickr, the target size is 337 x 500 pixels. That means a maximum height of 500 px and a maximum width of 337 px.
For a portrait format you should therefore enter 500 px in the height field and leave the width blank if you want the size to be relative to your original size.
For a landscape format you must enter a width of 337 px but leave your height blank.
Choose Save for Web to save your picture before uploading to web.
The Resolution field is relevant if you for example wish to resize your picture for print. Then you also want to specify your crop in centimetres.
Enter for example a width of 31cm and a heigth of 42 cm for a portait format, and a resolution of 300 dpi.
Cropping for Medium on Flickr will remove the possibility of viewing your image in a larger size on the web at a later stage, so make sure this is what you actually want.