Magic box

A "magic box" was a device which looked like a camera. It could create a model or alter the size of objects and emit a heat ray. The First Doctor used one to help Santa Claus make enough toys for Christmas, and to make models of the TARDIS, which many children had asked of Santa. The Doctor used a second box to shrink a polar bear, enlarge a squirrel, melt a living snowman and shrink the Demon Magician (TVC: A Christmas Story)

Behind the scenes
The Doctor's "magic box" is similar to the Master's Tissue Compression Eliminator in that it can shrink objects, except that it is non-lethal.