I've understood that an USSD message consists of 160 bytes. For 7 bit data coding schemes, the maximum number of characters is 160*8/7 which gives 182 characters. It's unclear to me what is the maximum number of characters for UCS2 encoding. Normally, it would be something like 160/2, but I have some mixed information on this.
The maximum size of a USSD message is 160 bytes. For GSM 7 Bit messages you are correct in saying the limit is 182 characters. UC2 encoding per definition is fixed 2 bytes per character so you will have a maximum of 80 characters.