Worst case is (factory OEM) seatpost at maximum extension, with maximum rider weight. The seatpost extension matters more than the rider height per se, but is due to rider height. Due to rider weight, that will exert worst-case aft moment loads on seatpost and seat tube, stress is an issue. Lateral loads there are not as big an issue. Pedaling loads will input to the frame slightly different based on seatpost extension and assuming a seated rider, but not as much a factor, because: The other frame tubes, handlepost, etc, are designed more based on minimum stiffness required, so stresses tend to not be max allowed for material.

In cyclic fatigue testing, there will be a lab test devised based on max weight and max peak G-loading, and may be a constant load cyclic test, or a histogram of loads. Sometimes this is calculated, sometimes the manufacturer has data from instrumenting a bike with accelerometers and riding it over roughest anticipated terrain. Lab tests are faster than road course testing, but are only valid if well-correlated with real-world tests. That data and correlation are closely guarded proprietary information. Which is why, when you see on aftermarket part packaging, "Meets or exceeds OEM specifications!", it's usually a lie, because that aftermarket maker does not have access to the OEM specs unless they are provided, such as if the OEM wants the aftermarket maker to produce service parts because the OEM doesn't, and they don't want their customers left high-and-dry because no repair parts available. Usually the aftermarket parts maker obtains the OEM part, and pulls it apart and copies as closely as possible, the dimensions, and material specs through material testing. The most difficult thing to copy are the manufacturing processes, which are not visible on the finished part.