According to the other feature request post at http://support.fancyproductdesigner.com/support/discussions/topics/5000051388, when fancy designs and user uploaded images are added to the Designer, they will be auto-scaled or auto fit as long as we set resize to width/height values in the main settings or individual product settings. However, currently auto-scale/auto-fit works only for user uploaded images, but not for Fancy Designs we add.
If I understand correctly, the only way to autofit Fancy Designs into Stage/Designer is to set Scale value in the Fancy Designs Options (Or Fancy Design category Options). However, after enabling the Fancy Designs (or Category) Options, we will have to provide all other settings (i.e., setting Bounding Box for each Fancy Design or Design Category using X,Y, Width/Height seems time-consuming and hard to maintain in the long run).
I have contacted the Support before posting this Feature Request; the Support team asked me to create this feature request for future improvement.