This page compares unicode encodings. Two situations are considered: 8 bit clean environments and environments like SMTP that only support 7 bit characters. SCSU and BOCU are excluded from the comparison tables because it is difficult to simply quantify thier size.
In summary
For 7 bit environments UTF-7 clearly wins over the combination of other unicode encodings with quoted printable or base64. For 8 bit clean environments things vary considerablly depending on what code points are in the text to be encoded.