Open the file in image ready> image>image size>put in the values.
If the scale isn't the same for what you want to resize (e.g say the image was 100*100 and you wanted to resize it to 200*200 the scale would be the same but if you wanted to resize it to 100*50 it wouldn't) then uncheck constrain proportions. However, if the scale isn't close to what it was before you resized the image could end up looking distorted.
Hope this helps