Well, First off, Borneo STP and Sumatran STP can get just as large as any red blood. I have personally seen a 6 foot Sumatran STP male and hes about 20lbs. It all really depends on how the animal is being fed and raised. I also feel genetics play a role in the size of the animal as well. If you get a baby that came from 5 foot long, and 20lb. parents, chances are that baby will get to be about the same.
With that said, you now know their are some exceptions.
Average brongermai male size is about 4-5 feet and about 15 lbs. I have a male sumatran right now that is over 5 foot in length and pushing 20lbs. I have a red blood male that is 4 feet and about 7 lbs. So size will vary, their is only averages to follow, and their is always an exception to the rule.
Bloods never stop growin, NEVER, accually no snakes stop growing to my knowledge.
If you are prepaired to house a 3-4 foot blood, you are ready to house a 5-6 foot blood, as they both require about the same sized encloser. So do your research before you buy an animal, get a cage ready and have fun... Bloods are by far the most amazing snakes to work with. Once you get one, you will get another... and another... and another.... and... you get the point.
Ask any questions you might have, its very important to get all of your information up front and their are LOTS of great blood keepers on this forum.
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Sam Craver
Cage Crafters and The Blood Bank