WPF Trigger when property value is greater than a certain amount

You can use a data trigger and set the binding RelativeSource to Self. Data Triggers allow binding and bindings lets you have converters.

Example:

   <Button Content="I change colour depending on my width for some reason">
        <Button.Triggers>
            <DataTrigger
                Binding="{Binding
                Path=Width,
                RelativeSource={RelativeSource Self},
                Converter={StaticResource isLessThanConverter},
                ConverterParameter=50}"
                Value="True">
                <Setter Property="Button.Background" Value="Red" />
            <DataTrigger>
        <Button.Triggers>
    <Button>

Reference