Many users are working in a CMS and have a custom format for their images. For example, images on a site all have 500 pixel width. In Clarkii you simply crop an area of the image down (or up) to 500 px width. You can do this with the manual crop option.
You get a crop frame that you can drag over the image and also change in size.
At the layer properties panel you will see the exact dimensions and position of the crop rectangle.
Also you can use Resample options tool to crop you image percent-wise.