The Fermilab Tevatron is currently the largest (6.28 Km in circumference) and highest-energy (about 1/7th of the LHC) running accelerator on earth. It will be second when LHC will get up to speed. Size wise LEP (which used to sit where the LHC is being built) detains the record as the largest accelerator with a 26.6 Km circumference (the same that the LHC will have). Oh another interesting fact: these devices often need to keep their magnets pretty cold (colder than outer space!) and use the la largest refrigerators on earth!