Two and one-half stars. Rated PG. 104 minutes. One of the best things about the 1988 comedy “Big” was Tom Hanks’ main character, a kid stuck in an adult’s body who used his playful imagination to rise through the ranks of a toy company. At one point, he pitches the idea of a robot toy that turns into a bug, arguing that kids will prefer something that’s both unexpected and fun. The premise of “Monster Trucks” is in that same spirit.