How can I photograph a bed so its width is obvious?

Using a wide lens is going to make the far end appear farther way, the lines to a vanishing point are at a sharp angle. The downside of this is it might make some people think the entirety of the apparent width is due to the very short lens itself, which could be counter-productive.
Looking at the lens options on that phone, the 23mm is a bit short, the 70mm is going to have you standing in the next room, so you might be stuck, unless there's a convenient door behind you. It still might look more 'compressed' than you'd like. A 35mm or 50mm would look more 'real'. Otherwise all you can do is use the 23mm & step back as far as you can, crop later.

You could just lay a human across the bed instead, maybe even corner to corner, so you can easily see where their feet are - of course, no-one's going to be able to tell at a glance whether they're 5'2" or 6'2" but it might give a hint. People are generally very good at determining the 'reality' of perspective from photos of humans.