Normally, the size of a product will depend on how it is packaged in the store. If any of the sides of the product exceeds 50 centimetres, the product is classed as “larger than 50 cm”.
Vacuum cleaners: Measure the largest part of the main body. Omit the tube.
Round object: Measure the diameter
Cables, string lights, etc: Measure the size of the rolled-up item, for example the box size
Foldable lamp: Measure the longest part.