I would do something like this on the wall back by what I think is a 3' window. I assume that is the entrance in the upper left. Turn it with the length of the bed on the same wall. This way you should have room for both a dresser/desk and a bedside table.

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It looks like a queen size to me. I love this idea of it being made up to function as a daybed. You could actually DIY the headboards and mount them to the wall rather than to the bed.

Well, because I am a hands-on type of person, use painters tape and tape it to the floor (the outline of the furniture), and see what feels the best for you to function in...