@Adrian2250 First off - it's nice that you've already set up a listing in anticipation of the room being complete, but I'd recommend keeping your calendar blocked and unbookable until it's ready. Any guest who books with you now might arrive with inaccurate expectations, having not had the chance to see the room and its dimensions.
Regarding bed size: in an Entire Home listing, bigger is often better - even if the bed cuts into the bedroom space, people have the whole rest of the house to sprawl out in. I don't find that this is the case for a Private Room listing. While some of your guests might choose to hang out in the common areas, many will prefer to keep to themselves in the bedroom, so you don't want the bed to make the rest of the room look cramped. The room has more value if there's plenty of space for guests to store their stuff, a desk/chair suitable for work, and enough floor space that people don't feel hemmed in. I would choose a bed frame that doesn't extend much beyond the mattress and ideally a raised one in which you can incorporate a built-in under-bed storage unit.
Not once has anyone mentioned a soft headboard as a thing they expected - good pillows are the key there. And bear in mind, only budget travelers choose rooms in shared homes - we care more about location and amenities than furniture design and fancy headboards.