To make plans from the picture you would scale measure the parts. The seat height would be 18" which would be a standard chair seat height. What I measure the back is 2.77 times the seat height making the back about 49". The width of the seat in the front would be around 15". That is kind of narrow but proportionally with the picture the seat width is less than the height. You might get some large paper and draw it out full scale to make adjustments on the parts before starting construction. The seat would probably be around 1 1/8" thick. The front legs at the top are probably 1 1/2" to 1 3/4" at the top tapering to about 1"x1" at the bottom. Of course because of the curve you would probably have to start with a blank 2" to 2 1/4" square and bandsaw the shape. The back legs on a chair like that are nearly always 1" thick. It's just flat wood that has the contour bandsawed to that shape. From the straight point where it fits to the seat it probably tapers back about 3" and likewise from the seat down. The top rail on the back you would have to lay that out once the back legs and the seat were made. You would need to follow the contour of the seat while making it terminate on top of the legs. Personally I don't like the design. I think the back legs should come up behind the top rail and screwed on from the back side. Having it dowel into the top of the legs makes it easy to break loose but then that would change the design of the chair. The rungs for the back are usually mortised into the rails. You could route slots into the rails for the rungs and then whittle the ends of the rungs to fit into the radius of the routed holes. If it were me I would build the chairs without the arms and once assembled lay out how to fit the parts for the arms. It would just dowel into the back legs and then down into the vertical support. The vertical support just use one dowel toward the bottom and once glued on drill a hole and put a screw in there too.
When you make up all the parts be sure everything is thoroughly sanded before you assemble it. It's really difficult to sand something like that after it's put together. You usually end up having to wash glue off with a wet rag which causes a place that needs sanding or get clamp marks in it but that is a lot easier to touch up some sanding than try to do all the sanding later.
