I believe that most jack posts are weight rated based on diameter of the post, diameter of the screw, and extension length (the shorter the screw the greater the weight load).
You should be able to measure all three aspects to see whats down there now.
If it's a case of just wanting to level up your floors it's less of a consideration.
If, however, you're attempting to provide structural support to an underbuilt or damaged house, then you really do want to consult an engineer.
My Mom's old house was built in 1903 and the living room floor had developed a sag as old houses are prone to doing, and the floor was swaying a bit when someone walked on it.
I determined the low point in the living room, then went to the basement and did some measuring across the sleepers. Essentially in was a case of the span being too long for the sleepers and over the years they had sagged a bit, so I got several jack posts and a couple of pieces of 2x6.
I scabbed the 2x6 together to form a beam, set it against the sleepers where I figured it would do the most good, and over the next several months very slowly raised the screws.
I made sure to measure at the ends of each end of the 2x6 beam to make sure that I was bringing the floor up evening.
It took about 6 months to raise the jacks to where where I wanted them to be. I raised them VERY slowly to allow the structure to acclimate to the changes and to hopefully prevent blowing plaster off the walls.
By the time I stopped, the floor in the living room was a LOT more even, the sag was pretty much gone, and the floor was no longer swaying when someone walked on it.
Just remember one rule of thumb... the more you extend the post, the less capacity rating it has.
One with a 4-8 to 8-4 adjustment range loses about half its load capacity when you get to 8 feet.