How to get WPF Window to autosize to content and no more

You need to use SizeToContent property, check the msdn link.

Example:

<Window xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        …
        SizeToContent="WidthAndHeight">