Pollen from both plants was the same size? This is a decisive factor.

A couple of things about the failure of your stellar cuttings:

1. did you allow the cutting ends to dry somewhat before inserting into the growing medium?

2. short sections are adequate for Pelargonium cuttings. Even a section with only a couple of leaves will do.

floral_uk: All pelargoniums (succulent or desert-dwellers set-aside) are dipped in rooting hormone and inserted immediately into potting medium. I had good success with standard potting soil as well as rock wool plugs. I did just few attempts with stellar hybrids. Main reason is their relatively slow growth, and often lack of availability. This time I'll let the two which I have (Vancouver Centennial) to grow till fall, and I will start taking cuttings. Do you suggest taking a longer cutting and still keep the two upper leaves?

Maritza: Did you mean diploid vs. haploid pollen? That's a critical issue, and if size comparison results from that, at-least I will have one easy way to distinguish the two. What is your magnifying glass power to make that easy to see? The market is flooded with tetraploid plants (which result in larger pelargoniums) and triploid plants (The latter are sterile and cannot produce seed). If stellars do not produce seed, I may suspect this is the case, but I cannot say that all stellars are triploid.

Asclepiad: I have read the stellar geranium does not give seeds always but in the continued crossing with the zonal you can get it. This is a process very long and very interesting. The book Geraniums III, writen by Mastalerz speaks about the hybridization. For to see the pollen's size is necessary a microscope. This was a experience to me because I wanted to cross two different geraniums.