The idea of the child in an adult's body (or adult in a child's body even) has been done in films before this, in films after this and will probably be done again in the future.
Whether "Big" is the best of the bunch is a pretty subjective matter but in my humble opinion it simply has to be.
Tom Hanks is simply wonderful as the 13 year old Josh Baskin and was quite worthy of his Oscar nomination. He brings an innocence to the role that is absolutely perfect, he never goes childishly silly and he never acts like a young boy pretending to be grown up, he simply acts like a 12 year old and the performance is incredibly realistic.
I think this is where this film scores over some of the other of this ilk, in that it concentrates on the golden times of childhood and doesn't become just a situation comedy. It reminds you of how wonderful it was to be 12, when the summer days seemed endless. It reminds you of how central your family was when you were young and how "best friends" really were the best.
Apart from Hanks there isn't really much room for anyone else. Robert Loggia as the kindly toy company boss is good value and Elizabeth Perkins as Hank's love interest performs with sensitivity. Special mention though for two fantastic child performers in both David Moscow as the young Josh but especially to Jared Rushton as Josh's best friend Billy.
It is sentimental, make no mistake about that, but I don't think it can be accused of being sloppy and I'm sure it will delight watchers both young and old.