This must be highligthed in the description of the item "Staff of the Python"

well, how big is "giant"???

It's Huge, so 3x3.

I mean in real terms, 20 ft long, 40 ft, 80 ft??

I think you're able to decide that however you want. I described it to my players as the size of the snakes in the movie Anaconda.

Thats normal size for it.

I would describe it as the size of the snake Conan, Arnold Schwarzenegger, fought in the pit.

 eeboleeeboleeeee

Why does this have an underwater environment tag when it cannot breathe underwater? I totally get the swim speed, just not the making its home on the bottom of a lake or ocean.

I did a bit of math a little, as I am using one of these (lightly modified into a warforged cyborg, of course) in my campaign.  The average boa constrictor is from 3-13 feet long, though length ranges of up to 18 feet have been recorded.  If a snake is approximately 7 inches wide at the widest point, and is 13 feet long, then a 7 foot wide snake would be about 140 feet long.  Granted, I do believe that a 7 foot wide snake, from tummy to back, would probably be considered a bit larger than Huge, that is a pretty decent ratio to think of it as...  The length being just over 22x the width of the snake, give or take, it can of course be up to 30x longer than it is wide.  So I guess a huge sized one would be maybe 4 feet wide and 88-120 feet long...  Also, since it has a bite attack, it is my humble opinion that the reach should be at least a quarter of its length, because snakes can strike in at least that range.

 if you make a hit dc success does it automaticaly grapple or does the enemy make a grapple check ?

I have this guy as a staff his name is Slinky and he's my best friend

If the attack hits then the target is automatically grappled. On the grappled creature's turn, it can use its action to try to escape by making an Athletics or Acrobatics check.

Can this breathe underwater?

I cant Believe that a staff makes this 

No, even though it is tagged as "Underwater", this creature cannot, although some species of snake do swim none (as far as I know) dive, so breathing underwater is evidently not necessary or useful to the snake in any way.

Can it constrict any creature? So if im fighting a dragon i can just use my Staff of the Python and constrict it?

Uh, isn't this a picture of a cat of some type?

I presume that until a creature breaks the grapple it takes that attack damage each round?

Quote from TheGux >>

Can it constrict any creature? So if im fighting a dragon i can just use my Staff of the Python and constrict it?

 It would have to hit, but yes. The DM might stop it though, since it would have to climb up onto a dragon, wrap around it, and not get thrown off. For instance, I'd be worried that the dragon would fly up really high and then throw it off, causing a significant amount of fall damage.