FEM Simulation Mesh Setting

Actually this is a very common issue I encountered every time I do simulation.
Anyway, I am using Catia 3D experience to do the simulation.

1. What is the best way to define mesh size. (mesh size smaller, more accurate result but longer simulation)

2. What is the best way to define mesh type. (2 type of mesh in Catia 3D experience: tetrahedron and octree tetrahedron, 2 type of order: linear and quadratic). I understand that tetrahedron and quadratic will yield the best result but it take too long. So how can I justify which type of setting I should use?

It will be the best if there's a Catia 3D experience user that can guide me :) Truly appreaciate it.