Most electricians would tell the size by looking at it , Brooke .
Presuming it is Red & Black . The proper way is what Sidewinder said . You could ask for a couple of crimp lugs at the wholesalers , a 6mm & a 10mm , switch off & try them on the cable ...see if one will slide on as a tight fit .
Or get a sparky in ....I'd guess you are limited to 30Amps ....probably looking at 7-8KW ...all guesswork ...get a man in .