Wow. I have absolutely no idea. I heard women's eyes dilate during ovulation. Do men ovulate? I should know that. Here's a body language site that claims our eyes dilate whenever we like someone, and contract when we don't. But keep in mind, that dilated guy might just "like you," and not "like you like you," and the pupil result would be the same.
This sounds like the kind of thing that might be true, but at the same time has no practical application. Why not? Because a guy's eyes will dilate if you enter a low light area, he's had coffee recently, or any number of reasons more common than his undying love for you.
Forget pupil dilation. If you want to know whether a guy likes you, there are much more regulated and easy to spot biological changes:
1. He gets an erection. Some risk of a false positive, as we tend to get erections if we sit the wrong way or a passing waitress reminds us of the chick from Species. But still, a better bet than pupil dilation. No one ever told their date "baby, you are making my pupils HUGE."
2. He hangs out with you, touches you, calls you...you know, dates you. Men are simple creatures. If we like you, you'll know it. Even if we're shy, we'll find underhanded ways to be in the same rooms as you as much as possible. If we're not doing any of that, we probably don't like you, no matter how raptor-esque our pupils become in your presence.
3. His Facebook relationship status is "it's complicated." I think this has been sufficiently assimilated into our lives to refer to it as a biological phenomenon, don't you? Which I believe technically makes us cyborgs. Awesome!
Next time you're staring deeply into a guy's eyes, try to enjoy the moment and forget about measuring his pupil width. All you'll do is creep him out, and then you'll be left alone with your shattered dreams and tiny eye tape measure.