Yes, there are people with one eye smaller than the other. I have a neighbor with one eyes smaller than the other. She is born with such eyes. But luckily, she has already made her two eyes the same size by take an eye surgery when she was 13 years old. Now, she looks the same as most of us. So, it is not strange for people with one eye smaller than the other.
Actually, everyone's eyes are not the same size. From the scientific theory, the face of everyone is not absolutely symmetrical. But this difference is hard to observe. And those who you find the difference of eyes have differed a lot in the size of eyes.