How big could an amphibian theoretically be in our modern days? In the past, down to the Mezozoic area there were amphibians the size of nile crocodiles but none after as it seems.
Was there something back than that allowed them to be this big or could they be much bigger today?

The giant salamanders (Cryptobranchidae) were the huge salamanders of the Mesozoic - three species in two genera are alive today, the hellbender (Cryptobranchus) lives in the USA but the two species of Andrias are bigger, with the Chinese salamander growing to about 2m (6'6") long.  They rarely get that big today though as they are eaten and used in Chinese medicine.