Is it possible to make giants plants or fruits by treating them with growth hormon ?

Not as far as I know. From what I know, most "giant" veggies and fruits are produced by inducing polyploidy (duplication of the entire genome) via colchicine.

Does Polyploïdy mean that the genome is copied inside the nucleus, or that several nuclei appear ?

The former - the cells look normal, with one nucleus, but have twice as many chromosomes as normal during division.

i know this sounds okey, but my grandparents have a lemon tree in their back yard. they both told each other that their not feeding it at all, because they supposedly had agreed on an organic means of growing food. secretely, they both wanted better and bigger stuff, so one would feed it alot of miracle grow in the mornign without the other knowing it, and same thing with the other at night. they got a lemon the near the size of a bowling ball! they made multiple lemong pies out of just that one lemon!