I think that in a general sense, they can be considered near-synonyms. I had to look up the possible differences between largo and praça and apparently, size is not the main factor. If it were, praça would win anyway, because you have very large praças, like Praça D. Pedro IV or Praça do Comércio, but all largos I can think of are medium to small, like Largo do Carmo or Largo de São Carlos (still speaking of Lisbon).
It seems that largo is a term that can be broadly used for any open space at an intersection/confluence between several streets. On the other hand, the word praça is a bit more specific and pressuposes that this open area (usually square) actively serves as a gathering place and that there was more urban planning involved in its construction.