Ok you have decided on an engine, so now look at what type of thrust it produces. Now you need to decide how many engines you want to use, or how much power you will have. Now you can rough out an idea of the max weight your airframe can be...
That will give you and idea of max size and type of construction methods to use...

The length, width, wings are all going to be limited on the amount of power you have to move it and the drag your airframe will produce, and the weight of the structure will give you an idea of how to size it.

The diameter of the Ducted Fan does not tell you much, what you need to know is how much thrust it will produce. How much weight are you going to need for the batt'. and esc's the weight of these will be part of the airframe weight you will have to consider in your design.

Look at
http://www.airfieldmodels.com
He has an series on how to design an aircraft, it may help you out.

One other sugestion, get a copy of Andy Lennons' book on model aircraft design, and read it before you start, you can find it a most hobby stores or an online book seller.