The piano song used in the film "Big" is "Heart and Soul". Tom Hanks as Joshua "Josh" Baskin meets Robert Loggia as Mr. MacMillan, the owner of a toy company (FAO Schwarz). Josh impresses him with his insight into current toys and his childlike enthusiasm. They play a duet on a foot-operated electronic keyboard, performing "Heart and Soul" and "Chopsticks". MacMillan then invites Josh to a massive marketing campaign meeting with senior executives of the toy company.

"Heart and Soul" is a well known song that was composed by Hoagy Carmichael with lyrics by Frank Loesser. It has been on music charts with different artists between the years of 1938 and 1961. The simplified version of the popular piano duet was used in "Big". The movie's duet is performed in front of customers in Schwarz and it appears that they are truly enjoying the live show that is being performed.

More Info: en.wikipedia.org