I don't keep great record of individual hen's output, so this is more a "feeling". I do have some stats buried in a spreadsheet I can refer to if I want to validate my impressions. I'm planning next year's flock now and I have noticed that our average hen lays about 3 eggs a week. Some breeds are better, I think the Rhodebars lay closer to 6 eggs a week (at least 5). I have 5 younger hens now (hatched last October at the end of the season) and I often get 5 eggs from them. Our "mixed breed" hens really shine, they are the backbone of our egg production and so I want to get more hybrids and more high producing strains to make further hybrids with.
My main concern is egg color (and number) more than egg size, but bigger is better of course. I want 3 main egg colors, light brown, dark brown, and blue. Some of my chick customers want white or green (olive or light green), but that is a much smaller market. So, for next year I hope to be producing mostly these breeds (all of which are sexable at hatch or a few days of age):
1. light brown - Rhodebars and Bielefelders, with most chicks being a hybrid of the 2 breeds, assuming they are still autosexing.
2. dark brown - Welsummers and Welbars, focus on Welbars because they are more sexable, but Welsummers are hugely popular, so I'll still have pure Welsummers.
3. blue - Legbars and Black Sexlinks, planning to switch from Barred Hollands to California Greys for the female side of the sexlinks, the BH are only average layers for me. I don't have a lot of experience with the Legbars, but hoping they produce as well as the Rhodebars.
A rather long-winded answer, I know. For dark eggs, I recommend Welsummers over Marans, but I have very high hopes for the hybrids I plan to be producing next year (Welbars and Silver Duckwing Welsummers). The parent hens I'm using now seem to be better layers than the pure Welsummers (prob from the small amount of Barred Rock in their parentage), but I only have 2 of those hens to compare with.
I will know a lot more this time next year (won't we all).