“pt” and “%” should also work. “pt” is converted on-a-fly to “px” bearing in mind 72dpi resolution. (optionally pd4ml.protectPhysicalUnitDimensions() API call suppresses the size conversions, but it has its drawbacks)
The issue you experience with font size is quite strange.
Try to temporarily define any other TTF font instead of HM Ampersand Ultra Light, i.e. Comic Sans MS. Is the problem still there?