Do you mean your Wii is set to 16:9 and you're trying to force the game to 4:3? Or is your Wii set to 4:3 and you meant to say you're trying to force the game to output at 16:9?
Unfortunately that's only possible with vWii.
Castlevania Rebirth requests IOS 53 and is compatible with IOS56, so as long as you're using the loaders default settings and you have d2x v11 beta 2 installed correctly then it should work with the auto setting.
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In general you shouldn't need to modify IOS settings, especially for WiiWare/VC. Although any game that might use IOS38 on Wii will need the upcoming d2x update.