What's the biggest size a cross can be without being considered a pectoral cross?

[Zacharias]

If you want a pectoral cross, buy one; it's no sin. Just be aware that if you wear it around outside your clothing, you will get some odd stares and questions, especially from other Orthodox. If that doesn't bother you, then go and do what you want.

I see your point. I guess wearing a big silver one could appear to be a "Look at me! Look at me!" and could become a prideful thing. I think I'll get the cheap wooden one.

Beware of getting caught up in man-made rules.

That's a great danger these days.