That'd be MM. Pretty sure roman numerals are case sensitive.
EDIT: also you didnt use a dollar sign so it was safe to assume you meant 5 millimeters. Pretty sure i just proved you wrong twice.
No three times i was proved wrong. The third being you we're capable of making the connection between the mm with the word dollars after it. That is my fault. I give too much credit. But nice work on google.
peace out I win!
logging off as you read this