When you select something that has transparency and you try to copy and paste it into a new document, it acts the same as if you were to do image > trim.
If you try pasting it into a new document, even if you make it the same size as the previous one, it will paste in the center of the document (I'm sure this isn't what you want).
What I have done in the past is to paint a couple of pixels in all four corners of the image you are trying to copy. This will make sure the image is pasted back into place where you want it.
Another thing you can do, kind of as a shortcut is to make a new layer under the layer you want 'copied' and fill it with any color. Once you have the two layers, you can select both and "convert to smart object." This will convert both layers to a smart object. You can then double click on that smart layer and it will open a new window with your transparency image and background image. Delete the background image and save the new image as whatever you want!