Wood type and measurements

Hi I am new to this forum. I have a question. I want to create my own wood cornice board/  with fabric and one without. I need help on how long do I make the board hanf the top of the window? and what type of wood should I use for a beginner like me? Is there any good instructional videos you can recommend me? I have 25 foot ceilings as well so Also need help on how high from the top of the window do I hang it? I also would like to add on curtains to the fabric window too. 

Thank you, jerma