Is it possible to estimate the step size and walking velocity of different person with the x-y-z acceleration data from 6 axial accelerometer sensor?
The answer—as with many such applied numerical problems—is, "sure, but you have to decide which of the published heuristics you want to use". Your real problem will be the embarrassment of riches as you try to choose a paper to implement or find an existing library implementation.

Using a quick Google search on estimating step size from accelerometer, I was able to find multiple articles proposing different methods for doing this. Apparently the research area is called Pedestrian Dead Reckoning (PDR), which makes sense since dead reckoning is the general term for estimating positioning using only velocity and acceleration data.

Some articles that attempt to estimate stride length as part of a PDR algorithm include:


Ngoc-Huynh Ho, Phuc Huu Truong, and Gu-Min Jeong (2016). Step-Detection and Adaptive Step-Length Estimation for Pedestrian Dead-Reckoning at Various Walking Speeds Using a Smartphone.
Haifeng Xing et al. (2017). Pedestrian Stride Length Estimation from IMU Measurements and ANN Based Algorithm.
Ahmad Abadleh et al. (2018). Step Detection Algorithm For Accurate Distance Estimation Using Dynamic Step Length.
etc.