resize to specific pixel-value and preview?

Is there any way to resize or crop an image to i.e. 400 x 400 px inside co9 and then look at the image at 100% at this exact size?

I wonder since I would like to use co9 sometimes as my only edit program and then export to jpeg and just use the image. Sometimes adjusments / edits on high resolution looks different after exporting at smaller width x height size to disk.