Looking at the first below I'd say, to be on the safe side, the bedhead should show half to three quarters above your tallest pillow. The second photo I believe shows too much bedhead.

You can have what ever ceiling height you can afford. But also think of what height windows, internal and external doors will be. What heating and cooling system you will install. What window treatments will be required for extra height windows, etc etc. Our double storey home has 2700mm ceilings throughout with 2400 doors and windows. This height is ideal. A gable ceiling in the living and dining areas, I think, has more character than a 5 metre ceiling in one room. Have 2700 ceilings throughout the remainder of home. Depends on the aesthetic you are going for, to what the design could be.

This wasn't your question, but looking at a possible boxed out window in the open plan kitchen dining living it shows just how limited you space is. The following 3D overview shows a boxed out window seat. I have run the kitchen along the same wall as the new window, The Island is 2m x 900 with 1m between the wall base cabinets and the island and 1m between the Island and the wall where your plan shows cabinets. If you have/had cabinets along the other wall as shown on your floor plan they will be 600 deep this would make the island 1400 x 900 which is small. Has your kitchen been renovated and were you considering that, if not. Looking at the overall size of your open plan area I would be considering 1 or 2 large stacker doors to open up to the rear alfresco, that's what I would do but you need to choose what will work for you PS I think that the boxed out window at the front of the house is a lovely idea but possibly not right when it faces West.

Thanks to everyone for your help once again. The drafting is not completed yet. It’s a custom house. Now, the builder is doing all the architectural work and will give us 3d animation and of course all the sides elevations. The roof angle is 25 degrees with 450mm eaves. The builder didn’t want to spend any time on this topic until we get the full house architectural drawings. The reason why I was seeking public opinions was to get an informed decision so that we can let them know what we really want now. We don’t want to delay our build because of change of mind on some minor things. Even though it may not have a significant effect, the part of the window directly under the eaves serves no purpose because the sun can’t reach it. If you are standing inside the house, and you are looking up through the window, you will see tunderneath the eaves (not good). This part is like a heat loss blackspot. That’s why I prefer to see 2 courses of bricks from outside. But I don’t know how this will turn out with high doors. I really want the doors and windows to be aligned through out the house.