I just did a search on the smallest croc/gator/caimans and here is the result, bear in mind that they don't tame well.
Smallest alligator, Alligator sinesnsis; 2m max in historic times, CITES app.1, less than 200 in wild remain. More exsist in captive populations. CHINESE ALLIGATOR
Smallest crocodile, Osteolamus tetraspis and it's sub species, O.t. Osborni; both CITES app.1, threatened, 1.2m ave., some males have reached 1.8m. DWARF WEST AFRICAN, and DWARF CONGO CROCODILES.
Smallest caiman, Paleosuchus palpebrosus, CITES app.2, threatened, 1.2m ave., 1.6m max.. CUVIER'S DWARF CAIMAN.
The only one you have a shot at legally owning is the caiman and it will not be cheap or easy. Or save time,money and effort by getting a salty, it will grow large quickly then eat you, heheh.