I had one (and she was even smaller than the others!) that did the same thing! She pecked at feet and eyes -- I don't think it was that she was mean (even though she now has the nickname of Ms. MEANY) but you could just see the curiosity in her every time another chick's eye got close.... she had to peck it! I thought she might have pecked out one eye each on 2 of my other chickies (they walked around with one eye shut for a day and a half or so). Finally they did heal and open their eyes back up.
So, I sectioned off part of the brooder with cardboard and put her in there with her own water and food and separated her for almost a week! I would try every morning to let her join the others and see what would happen. She would go after the feet and the eyes every time. One time I saw her peck at another chick's feet, grab a toe and hold on and drag the chick from one end of the brooder to the other.
Anyway, what ended up happening -- I went in there one morning and looked into the brooder and she wasn't in there! Actually, she wasn't on her side.... she had flown up and over the cardboard partition and joined her sisters on the other side. Well, ever since, she's been doing fine with them and hasn't been pecking her sisters' eyes anymore.
But for now, separate them... that's what I did. I put her in chickie jail for a while. Also, go and buy a red bulb. I did that and it instantly calmed all of my chicks down! I couldn't get over the difference in how excitable they were with the regular bulb versus how calm they were with the red bulb.