What is the size of a Coenzyme Q10 (ubiquinone) molecule?  (Note: I'm
just looking for the size of the molecule, not the molecular weight or
particle size.)

Ubiquinone:  Molecular Formula  C59H90O4 

I'm not sure what you're asking, by "size."  I'm assuming you mean
perhaps its dimensions.  Its primary "folded" shape is approximately
21 Angstroms in length.  The reference for this is below:

Lenaz, Giorgio. (1999) Localization and mobility of coenzyme Q in
lipid bilayers and membranes. In: Lenaz, ed. Biofactors 9. IOS Press;
pp87-93.


This website also shows a drawing of the molecule.  

http://www.sigmaaldrich.com/catalog/search/ProductDetail/SIGMA/C9538


Hope that helps!

-GreyW01f