i\'m designing a cross platform app using xamarin forms. every page/view/Form designed from code behind. now i want to read Height and Width of the device used by user. base
In Xamarin Forms Labs here there are classes and examples for getting Device screen information like what you are after.
There are some further notes on implementing this and getting the Device object that you require here.
On a different note, if your only on about placing Headers and Footers then why not use the inbuilt Xamarin.Forms controls to auto-expand controls and layouts etc, that will adapt automatically based to screen of the user's device?
I get the impression that you are looking to go down an AbsoluteLayout approach and specify values yourself? If so, there really is no need. Especially for Headers and Footers of a Layout?