Wouldn't armor spikes be sized for the armor you're attaching them to ? Can you wear oversized armor ? I wouldn't allow it, personnally, but I'm not your GM.
If I were your GM I would allow it, but the size penalty would apply to all attack rolls you make, not just armor Sprite ones, because the giant spikes would be in your way. I'd also apply it to all skills that are affected by an ACP.