I wish I could remember what site showed the steps for making the ornament covers, if I do I will repost for you. For now take a paper tape measure and measure the circumference of the ornament. Then go and buy some styrofoam balls in that size while wet pin the fsl to the styrofoam it will dry in the shape of the balls. Hope this helps.

As MissCharlie said, I believe you measure around the bulb. Embroidery Library may have the information you are after.

Here is a link with instructions.