In order to capture all necessary geometric details, it can be necessary to add refinements, especially if there are parts which are rather small in relation to the whole assembly. Also, regions where physical effects need to be covered well need to have refinements. This includes the inlets and outlets of a flow domain.
This article describes how to estimate the correct cell size before a mesh is generated on the example of a heat sink with thin ribs, as shown in the following figure:
You can get an impression of the dimensions of the geometry with the scale in the workbench:
Zoom to the part you are interested in and measure its thickness, then divide it by the numbers of cells required to define the cell refinement. In this case, the ribs have a thickness of 0.002 m.
For thin parts like the ribs of the heat sink, it is recommended to divide them in at least two to three cells. In this case we divided them into seven parts.
Accordingly we need to divide the measured 0.002 m by 7 which leads to a cell size of approximately 0.00029 m.