If you're designing your box around your moulding, the length of it is the easy part. Allow at least 3" on each end for packing.
If you're wondering about girth...consider selling your moulding in boxes that will easily accommodate a couple hundred of feet or less. You don't want your moulding moving around much otherwise it'll scar. A tight box ships best. Consider that people may stack your boxes and a box that is wider may be better.
If you're shipping in smaller quantities, either plan on purchasing some cheap packaging material or have small diameter boxes made.
Also consider stock sizes from your local box manufacturer. If they're already doing something similar, it can mean a big break for you.