"How would I know how much bigger area should be used near the cross section and when should this enlargement of the cross section area begin (I mean at which length)."
You would start with an initial design concept, such as the rod flexure example I provided above, build a FEM of the initial design, look at the results and come up a list of changes that become the first design iteration. Update the FEM, look at the results, come up with more changes that become the second design iteration. Continue iterating until the design meets all the requirements, or is optimized for some metric such as minimum mass. The iterations can be manual or you can use some of the tools Ansys provides to automate the iteration.