I'm thinking about starting to use Xamarin, is it possible to take a full-screen WPF project and use it on iOS and Android ?
No. Xamarin.Android and Xamarin.iOS expose the UI of the relevant platforms - you can't just use WPF on them.
However, if you separate out your "business logic" from the UI logic in your application, you may well be able to use the same business logic project, and just create new UI projects (one per platform). Of course, that relies on your "business logic" only requiring functionality which is available on Xamarin; more generally you'd want to create a Portable Class Library with that code in.
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