,but you are sizing the bolts depending upon the axial forces, shear forces, bending moments (and maybe torque as well). So the sizes are dependent on these values. If you only consider the axial and shear forces, then size is going to be smaller than the size if you also take into account the bending moment (and maybe torque).
I mean its a simple subject, I don't know why are you getting confused. It proceeds like this: You model the bolt as line body in the ANSYS (if you are only interested in finding the max forces and moments at any cross section of the bolt), then you determine the diameter of the bolt depending upon these values. You have to use an appropriate failure theory; for ductile materials you can use Tresca or Von-Mises but for brittle materials you can use Columb-Mohr or Modified Mohr. I usually use Von-Mises for ductile and Modifield Mohr for brittle since they agree to what we see in reality. You can also use a FOS as a part of your sizing and I usually use 1.5.
You can also use solid bodies to model the bolt in your model in ANSYS but that doesn't bring any benefit unless your purpose of analysis is not to size the bolt but analyze an already sized bolt that it will fail or not.