1 inch is 2,54cm.
Or just google for a converter, they're easy to find.
Ceramic isn't bad for orchids, it's just different. Plastic holds moisture longer than clay does since the clay breathes a bit. So for moisture loving plants it means watering more, but in an orchidarium that isn't much of a problem. Your culture (light, temp, fert..) is going to affect the plants a whole lot more than the pot material. But I don't really find much of a difference between the two. For now I prefer plastic because it's lighter (I'm traveling between NL and FR quite a lot, so weight matters!)