An average adult sized colon is around 5 feet long inside the body.
So that means that the colon could cold a lot of poop before you pooped it out but usually you would poop the poop out of your colon before it got too full unless you become constipated.
Your colon and body can hold up to 7 to 8 lbs of poop inside it before you get completely blocked up.
I would not recommend it but you could hold your poop back for as long as possible and then weigh yourself and then after you poop you can then weigh yourself again and see how much weight you lost.
Then that would tell you how much poop you had in your body.
When a person has poop in their body they usually produce between 14 to 17 ounces of poop per day that they poop.
So overtime that could build up to the weight I listed above but you don't want to wait until your poop is filled up before releasing it from your colon because that can cause health issues.