Newbie : need help to do a mosaic from tiles

[franco1000]

Hello ,
I moved layer A partially over the Layer B: They are superimposed so I must crop a slice of A ( or B). I tryed to make a python script using the method " pdb.gimp_layer_resize".
But i had only bizzare results not corresponding to the instruction of the Python console help.
A bug exists, i don't know if in the procedure or in me.
Some suggestions?

Thank you,
Franco