"Tener la mano larga" = To have a long hand*

To have a tendency to hit others very quickly. To be prone to use physical contact.
"¡Vaya con el niño! !Qué mano más larga tiene!"
"Deja de pegar a tus hermanos. No se puede tener la mano tan larga."
A second meaning would be to have sticky fingers.

In some South American countries the expresssion is also used to refer to thiefs as well as men who like touching women without their consent.