Actually, I think I ruled them too big. Going by the chart on page 136 of the PH, a chair is small, a tome is tiny, a scroll is diminutive, and a potion is fine. I think a medium-size creature's amulet wouldn't be any larger than a scroll (possibly even a potion), and a helm is much smaller than a chair.
Maybe the ACs should look something like this (assume no deflection bonus)...
Amulet: 5 (example Dex mod of wearer) + 4 (diminutive size) = AC 9
Helm: 5 (example Dex mod of wearer) + 2 (tiny size) = AC 7
That seems awfully low. Maybe they should look like this?
Amulet: 5 (example Dex mod of wearer) + 8 (fine size) = AC 13
Helm: 5 (example Dex mod of wearer) + 4 (diminutive size) = AC 9
I don't know. What do you guys think?