No, the trim around windows and doors. Sometimes when people measure a space they forget that they probably don't want the sofa to overlap the window trim, or to be above the window sill.

Make sure your furniture will actually fit in took the room too, so know how wide you doorways are. The place you buy from should be a le to tell you if a piece will fit through a door the size of yours. But remember, it doesn't necessarily need to come through the front door!

Consider doing an inspiration board for yourself. Clip out pictures of furniture, rooms, colors etc. of what you like. Then post a thumbnail sketch of the floor plan with dimensions so we can suggest a furniture plan. I always find having a "Master Plan" keeps you on track with the design concept and you will know what sizes you need.

When doing the thumbnail sketch: First take overall measurements of the room. Then start in a corner and go all the way around the room without missing any measurements: Corner > window including molding > to corner > wall to opening, etc. You get the idea.

Also post some pics entering the room, looking back and anything else you think is important.

I would be happy to do a floor plan for you and love to help.

Since you are working on a blank canvas I personally would begin by choosing a rug then go with the style of it to choose a sofa and chairs, pick colours from the rug to buy pillows and fabrics if you intend to have curtains made next is art this is a personal taste only but try to follow the style of everything else .

I think it's good to first start with developing a vision for your home. Usually that vision is inspired by something special (a specific) - the house itself, the land on which it sits, the neighborhood, a piece of furniture, a view, a meaningful painting or collection, a vintage area rug or textile - or a feeling. Sometimes not. Unfortunately, the homes that are not inspired tend to lack vision, and while attractive, they lack soul.

With that said, my advice is to develop a clear vision for your home. Once that is done, the specifics have an uncanny way of coming seamlessly together.

Denise's suggestion of creating an inspiration board is an excellent tool in honing your vision.