If you look at pictures of adult welsummer the roosters are a darker and different color than the hens, so as they start to get their feathers they will come in darker on a rooster, especially around the neck and chest. I can usually tell sexes apart by 5-8 weeks based on comb color and...
At about 4-6 weeks most roosters will get a pink comb. The pullets will stay a orange yellow color. Size doesn't matter as much as color. Leghorns have a bigger comb. Welsummer feather out differently between the males and females. So mostly you need to wait and watch for those pink combs.
That's way more than mine had today. Mostly sex links, Australorp, leghorn, and "Ameraucana", and a bunch of ducks. I certainly hope it is a barnevelder for you. I really like mine, they are pretty.