Red Sexlinks TEND to lay bigger eggs than Dominiques, but this is by no means a concrete rule. You'll have a tough time telling the eggs apart, unless you get to know the individual hen's eggs (size, shape, shade of brown, speckles, etc.).
I know what the hatchery websites say, but I was hoping to hear from someone with actual experience with dominiques. Sometimes the two types of sources are not in complete agreement.