Convert System.Drawing.Font.Size to WPF FontSize
By multiplication. A point is 1/72th of an inch, whereas WPF device-independent units ("WPF pixels") are 1/96th of an inch.
You can verify this by specifying a WPF control's FontSize property in XAML as for example "24" and "18pt". You will realize that both values result in the same actual font size.