Problem: Cylinder appears to be block

I am quite new to this program and I tried to do create a 3D circular duct.

I selected Cylinder and chose a radius of 0.004 m. In case of a short height compared to the radius, the cylinder keeps its shape in the graphics window. But when I select my desired height of 2.6 m the shape of th cylinder turns into a block! I found out that, the more the height/radius ratio is increasing, the less round the profile becomes.

Is this because COMSOL can't simulate this profile? Or is it just a bug?

I need to create a 3D object, because it appears that COMSOL does not give a correct solution using a rotated 2D object..

Thanks in advance,
Fabian