Is that an optional wall they are displaying on the floor plan? And then a sliding door you might add?
If only they made the length of the bedroom shorter in order to make the width of the den wider. I think closing in the den will make the entire space have a more closed in feel. I might choose the optional wall so that you then have the option of a sliding door should you need it for a second bedroom. But I think this unit is best as a large 1 bedroom+den.
If you were to add a twin bed, I wouldn't squeeze it into the width of the room. I'd place it in the bottom right hand corner in correlation with the dimensions of the room.
I thought to legally call a room a "bedroom" there had to be a natural source of light. So if you are trying to sell or market this unit as a 2 bedroom by closing off the den, you might want to check with a real esate lawyer on this point.