Vent sexing is only guaranteed 90%, you should always have a Plan B for any unwanted roosters whenever you order chicks. But size may not be an indicator of gender. The first signs are usually redder combs that are larger than the others'. Their wattles often begin to show sooner & redder too. I think they look cute like that, like gawky young teenagers sprouting their first beard hairs. By about 9 weeks you may see their pointy hackle (neck) and sadddle (in front of the tail) feathers beginning to sprout, part their feathers at those places & look for them emerging from their skin.
Fertilized eggs look & taste like unfertilized eggs. They don't begin to develop unless they were kept under constant moist heat, conditions you'd find in an incubator or under a setting broody hen. They won't begin to develop in a nest box shared by many hens, or on the counter, or even if left accidently in the nest for a few days.
Many people like keeping a roo or two with their laying flock, a good ratio is 1:10-12 hens.
