Our Marans are quite light. The size comes from the hens and the colour from the cock, so breeding with him won't necessarily get large birds. It would be wise to test his fertility before embarking on a breeding programme. I previously used to look at the blastoderm on the surface of the yolk, but have now discovered that method is unreliable, having had what appeared to be fertile eggs produce absolutely nothing (93 eggs) and had to replace the cock. The only reliable method is to incubate some and candle at day 8.