I heard that the bhp reaches a size of 5 m +, is that correct ???

[lilik hendrayana]

and how much maximum weight can be obtained from black headed python

 

[Dragoon]

I dont know the exact size but the QLD bhp are known to be the biggest

 

[Sdaji]

No, they never get that large. They are fairly large pythons, they sometimes will exceed 3 metres, but certainly not 5.

If you really wanted to you could get a large one extremely fat and heavy. It would be ridiculous and cruel to the snake.

 

[Pauls_Pythons]

I think someone was telling you porkies. My biggest girl would be around the 3.5 mark.
As for maximum weight, as Sdaji says above you can over feed one to make them as heavy as you want but it wouldn't be a good thing to do and would most certainly lead to an early death.