Binding to Self/'this' in XAML

First use a comma between the RelativeSource and Path in your Binding:

<Button x:Name="button1" Width="{Binding RelativeSource={RelativeSource Self}, 
                                Path=ButtonWidth}"/> 

Secondly, the RelativeSource binds to the Button. Button has no property called ButtonWidth. I am guessing you need to Bind to your parent control.

So try this RelativeSource binding:

<Button x:Name="button1" Width="{Binding RelativeSource=
    {RelativeSource FindAncestor, AncestorType={x:Type YourNamespace:YourParentControl}}, 
    Path=ButtonWidth}"/> 

I think what you are looking for is this:

<Window x:Class = "blah blah all the regular stuff"

DataContext="{Binding RelativeSource={RelativeSource Self}}"

>