When I increase the size of the font used in the title bar, the size of the title bar doesn't increase together. So, if I choose a font of size 18 I can only see half of the title. It gets cut horizontally.
Yes, this is currently a limitation of the Xfwm4 theming mechanism, I don't think this will be resolved.
Instead with Xfce 4.16 we will switch to using Client-side decorations more which don't expose this problem at all.