When creating a new face object (e.g., with FT_New_Face), an FT_Size object is automatically created and used to store all pixel-size dependent information, available in the ‘face->size’ field.
It is however possible to create more sizes for a given face, mostly in order to manage several character pixel sizes of the same font family and style. See FT_New_Size and FT_Done_Size.
Note that FT_Set_Pixel_Sizes and FT_Set_Char_Size only modify the contents of the current ‘active’ size; you thus need to use FT_Activate_Size to change it.
99% of applications won't need the functions provided here, especially if they use the caching sub-system, so be cautious when using these.