A product packaged in such quantity that proper lifting techniques are required to avoid a debilitating injury.
Common "Lift with your legs"-sized products include soda, M&Ms, and pretty much anything that requires a club store membership to purchase.
Sizes larger than this include "Team lift" size, and "It comes with wheels."
"Where did you get this 'Lift with your legs'-sized tub of cheese balls?"
"Sam's Club. I had to use a back support belt to load it into the back of my truck"