The maximum size for single custom SMS messages is 160 bytes. This translates to 140 ASCII characters or 70 Unicode characters.
Even if only a couple of the characters are UNICODE, the entire message will be encoded.
NOTE: The content should be kept within 160 GSM, 140 ASCII or 70 Unicode Characters per single message. If the content combines GSM with any character from ASCII or Unicode, then the length should be 140 or 70 characters respectively.
There are some exceptions where destinations have compliance requirements that may reduce the characters available.