A variable-size shelf for a refrigerating appliance, said variable-size shelf comprising a U-shaped frame on which a first shelf portion and a second shelf portion are assembled, wherein: • said first shelf portion and said second shelf portion comprise respective trim pieces (221, 222, 231, 232) mounted along their side edges and configured to allow their assembly with said U-shaped frame, • the first shelf portion is hinged on side members (212, 213) of the U-shaped frame close to a rear member thereof, said rear member being intended to be arranged at a back wall of a refrigerator compartment of a refrigerator appliance, • the first shelf portion can be rotated between a horizontal position and a vertical position, • the second shelf portion is slidably arranged on the side members (212, 213) of the U-shaped frame and can be moved from an extracted position, where it is substantially co-planar with the first shelf portion, and a retracted position, where it is received under the first shelf portion and is coupled thereto by way of guides formed in its trim pieces (221, 222), • the rear member and the side members (212, 213) of the U-shaped frame are manufactured as individual members, • the rear member and the side members (212, 213) of the U-shaped frame are assembled to one another by way of coupling means, • a front half of each side member (212, 213) of the U-shaped frame comprises a longitudinal protruding rib configured to slidably receive and support an edge of the second shelf portion in the extracted position and • said protruding rib comprises a stop member formed close to the free end of the side member, said stop member being configured to engage and hold a recess formed in the trim piece of the second shelf portion in the extracted position, characterised in that the end of the protruding rib facing the rear member of the U-shaped frame in an assembled configuration comprises a notch configured to receive a hook formed in the trim piece of the second shelf portion. • said first shelf portion and said second shelf portion comprise respective trim pieces (221, 222, 231, 232) mounted along their side edges and configured to allow their assembly with said U-shaped frame, • the first shelf portion is hinged on side members (212, 213) of the U-shaped frame close to a rear member thereof, said rear member being intended to be arranged at a back wall of a refrigerator compartment of a refrigerator appliance, • the first shelf portion can be rotated between a horizontal position and a vertical position, • the second shelf portion is slidably arranged on the side members (212, 213) of the U-shaped frame and can be moved from an extracted position, where it is substantially co-planar with the first shelf portion, and a retracted position, where it is received under the first shelf portion and is coupled thereto by way of guides formed in its trim pieces (221, 222), • the rear member and the side members (212, 213) of the U-shaped frame are manufactured as individual members, • the rear member and the side members (212, 213) of the U-shaped frame are assembled to one another by way of coupling means, • a front half of each side member (212, 213) of the U-shaped frame comprises a longitudinal protruding rib configured to slidably receive and support an edge of the second shelf portion in the extracted position and • said protruding rib comprises a stop member formed close to the free end of the side member, said stop member being configured to engage and hold a recess formed in the trim piece of the second shelf portion in the extracted position,