VSTO and WPF with High DPI results in small fonts

I'm currently creating a VSTO AddIn for Outlook 2013 with Custom Task Panes, WPF, C#, .NET 4.5

For the custom task pane I'm using a WinForms UserControl that hosts a WPF UserControl. It works fine on default DPI setting, but I also have a notebook with QHD+ 3200x1800 pixel: Images and for example the rectangle of a CheckBox seems to have a correctly scaled size, but the font is too small.

I'm have the WinForms UserControl AutoScale to Inherit and tried all other options with no effect.

I don't have a font size specified - everything is default

Thanks for your help.

Regards,

Bernhard