I have both Black Copper and Wheaten Marans. My Marans are not at all flighty or aggressive. My Marans lay large eggs in good numbers which are a dark brown.
I have no experience with Welsummers, as to my taste their are better alternatives.
Easter eggers are ok but generally have a less than attractive appearance in my opinion. I have some that I am using in a breeding project, and they lay a large green egg. the other side of the cross having been light brown layers.
Polish can be good white egg layers. They have a reputation of being flighty, but I suspect that is due to most people not trimming the crest feathers which results in limited vision and a propensity to being suddenly startled.
Minorcas are also a good choice for a white egg layer.
There are many breeds that lay large light brown eggs in good numbers.
As far as coop size the general recommendation is 4 square feet per bird for large fowl. The more room you can allow the better. I have 12 hens in one 8x12 coop with 4 nest boxes, but generally they only use 2 of the nest boxes.
When building a coop it is wise to consider ease of upkeep and maintenance in daily chicken keeping. It should be chicken keeper friendly.
