Can you use an iPhone charger for iPad and vice versa?

I know you are physically able to because I saw my iPad charge, but will any damage come onto the battery or device? If the chargers are the same, why is the iPhone charger much smaller than the iPad one?

You can do it vice versa, but the ipad charger is more powerful and is made for the I pad. It will charge the ipad faster than how fast the iPhone charger will.

It is ok to use iPhone charger for iPad or vice versa, but use iphone charger for iPad need to take longer time to full charger than iPad charger.

Yes you could
No it wouldn't

If the chargers are the same, why is the iPhone charger much smaller than the iPad one?
---I assume in that case you are referring to the (newer) iPhone's Lightning cable vs the older 30-pin connector. Of course in that case you can't use one to charge the other.