Hello Dilhar, to answer your question, there are many answers to your question. Are you designing a bedsheet pattern that is screen printed or jacquard woven? If it is screen printed, is it to be flatbed printed to the full sheet size or rotary printed? All methods of manufacturing will dictate the parameters of your Photoshop canvas size, and each manufacturer has their own set or parameters. If you are only designing concepts, not manufacture-ready design files, size your canvas to the best aesthetic outcome of the artwork.
Hello! I recommend flattening all the layers before offsetting, this will contain your visible image to the canvas size. If the layers are not flat, the layer size can and maybe larger than your canvas size. In this case the result would be offsetting a layer that is larger (or smaller) than your canvas size. I hope this helps. - Jeremy