Can I use 320X50 banner size for iAd on iPad?

It seems if I use iPad, the bann开发者_开发技巧er size is set as 768X66 by default. Can I use a smaller size like 320X50?

It's not the "default", it's the only supported size for the iPad in portrait orientations. See here.

If you're asking if you can simply display a smaller ad banner, the answer is no.

It's possible that your 320x50 banner will be scaled up to 768x66, but I don't have any actual evidence of that being the case. Either way, you will likely run into problems with getting your app approved for the App Store if you don't meet the explicitly stated requirements—Apple is picky about these things. Why are you averse to creating a larger side banner?

Also consider that you really should support landscape mode on the iPad, which would require a banner sized 1024x66, according to the above link. Even if your 320x50 banner is scaled, it's going to look terrible scaled to that size.