Size does matter: "Little Man" is big on gross-out humor and slapsticky sight gags that appeal to the lowest common denominator, but small on genuinely clever laughs.

Marlon Wayans, technologically manipulated to play a pint-size jewel thief who pretends to be a baby, does look ridiculous in his onesies and matching beanies, which is good for a guffaw here and there. But you can only get so much mileage out of that image, even over a film that's under 90 minutes long (but still feels interminable).

Marlon and his brother/co-star, Shawn, co-wrote the script with brother Keenen Ivory Wayans, who also directs. So if you've seen any of the family's other films ("Scary Movie," "White Chicks"), you know exactly what you're in for: boob jokes, poop jokes, penis jokes, jokes about getting kneed/hit/kicked in the groin.

It's juvenile and repetitive but not all that offensive, until Marlon's character, Calvin Sims, gets pummeled during a beer-soaked arena brawl by a professional hockey player who truly believes he's a baby. That's when you can put away the cake and send home the dinosaur costume guy -- the kiddie birthday party is over.

Calvin ends up in this infantilized state after pulling a jewelry store robbery with his partner, wannabe rapper Percy P (Tracy Morgan, making fun of Master P). The two have just stolen a $100,000 diamond at the request of a Chicago mob boss (Chazz Palminteri, eternally stuck in the same role), but with police chasing them, Calvin drops the stone into a purse belonging to up-and-coming ad executive Vanessa Edwards (Kerry Washington, a long way from "Ray").

Vanessa and her husband, Darryl (Shawn Wayans), had just been talking about whether they were ready to start a family, which gives Percy the idea to dress Calvin up as a baby and sneak him into the house to steal the diamond back. Logical, right?

We never see the transformation take place -- and we should have, because it actually could have been funny -- but all of a sudden, Calvin is lying in a basket on the front porch of Darryl and Vanessa's suburban home, dressed in a diaper and bonnet and swaddled in a pastel blanket. (Naturally, in the three seconds before Darryl steps outside and finds him, a dog lifts his leg to pee on Calvin.)

All the adults are complete idiots, of course, because no one seems to notice or care about how freakish baby Calvin is -- the fact that he has a full set of teeth, for example -- not even the doctor who examines him. The only one who's onto him is Vanessa's cantankerous dad (comedy veteran John Witherspoon), whom everyone assumes is senile.

Nonetheless, the couple takes him in for the weekend. Hijinks ensue, including nipple biting and violently soiled diapers. In one of the more disturbing sequences, Calvin not only watches Vanessa and Darryl having sex from the railing of his crib, he later joins them in bed.

In this instance, apparently size doesn't matter.

"Little Man," a Columbia Pictures release, is rated PG-13 for crude and sexual humor throughout, language and brief drug references.