I layout my labels using my iPad and and a photo editing app called art studio. Art studio allows you to resize/crop your “canvas” based on pixel sizes. I convert my measurements from inches to pixels and layout my label. Once that is done, I use Avery design pro to print. Avery Design pro allows you to size the image in inches but does not maintain the aspect ratio, that’s why I do it in two steps.
I created a spreadsheet with label and laser engraver sizes for each pen that I carry in my inventory. Length is easy; for width it's (tube diameter x pi) for labels, and (bushing diameter x pi) for engraving. When sizing for the engraver, if the bushing diameter is different on the ends of the tube I use an average. Also, in addition to molds, the nice folks at PtownSubbie sell clear acrylic label sizing templates for a bunch of different pens.