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The dimensions of a sprite are refered to in the basestats of each Pokémon, i.e. 5 by 5 tiles for abra. When replacing the png-file with a 7 by 7 tile picture and making no alterations in the basestats.asm-file, pokered.gbc is put together without any errors and the larger sprite can be loaded correctly. Do I really need the info (in the basestat-file) in the first place or are the dimensions of each *.PNG to be included evaluated at some point of the reassembly routine?
The dimensions of the pic are included as the first byte (55, 66, 77) of the compressed pic in the game's compression format. Although, this byte is rarely (or never?) used by the game.
Like you said, the dimensions are grabbed from the base stats rather than grabbed from the first byte of the pic.
To make this easier, we edited the base stats files to dump the first byte of the pic file.
In the example of Abra, data/baseStats/abra.asm includes this line:
INCBIN "pic/bmon/abra.pic",0,1 ; 55, sprite dimensions
This way, if you replace the Abra pic and give it new dimensions, the base stats file will be correctly updated.