When exporting an STL from Fusion 360, one must select an STL refinement level to use for calculating the maximum triangle count.
For FDM printing (0.05mm and above layer heights), where is the point of diminishing returns on STL refinement level when printing PLA and PETG on an Ender 3 with a 0.4mm nozzle? All mechanical components on the printer are stock.
I don't know that this can be definitively answered for a specific printer and all arbitrary designs.
The refinement level basically determines how smooth a curved surface will turn out. The STL file format can only express an object in terms of triangular-shaped surfaces, so Fusion 360 will need to approximate a curved surface by breaking it up into triangles. Flat surfaces and straight edges can be represented perfectly, so they won't be affected. Low refinement will use a small number of relatively large triangles. On a part like your example, the cylindrical shaft will have noticeable facets. Higher refinement means a larger number of smaller triangles.
If you have "Preview Mesh" checked as shown, you will be able to see the triangle wireframe, and you can use your own judgment if it's "good enough".
Ultimately, higher refinement means longer processing times and larger file sizes. The final print time won't be affected much if any.
Personally, I always use high refinement. Even on my modest system, it only takes a few more seconds to prepare a multi-hour print, and maybe a few hundred kilobytes or a couple megabytes on my hard drive that I will barely notice. This is a small tradeoff to ensure the best possible STL definition.
STL file size should not matter these days as far as the memory and storage sizes are concerned. These are generally large enough (unless you have very large and complicated STL files of large dimensions), so saving an STL in low or high resolution should usually be a matter of megabytes. However, the higher the resolution the more processor calculation time is needed.
Fusion 360 defines the STL generation through 3 predefined settings from a coarse to a fine setting. Indeed, coarse settings may result in poor resolution regarding curved surfaces. If the resolution is more coarse than the print resolution, you will definitely see this in your sliced print as you can identify the triangles of which the STL file consists of.
As far as there is no clear objective point of diminishing returns, the question cannot be answered. But, slicing with a higher resolution than print resolution does not seem sensible, it creates larger files which cannot be printed to that level of detail. But there is no downfall other than larger storage space and computation speed.
If you print for example a thin wall cylinder vertically you cannot turn the refinement high enough to not see the flats on the surface. My triangle counts are 524 for Low, 828 for Medium, 2206 for High, and if you select custom and pull the surface deviation slider all the way to the left you get 6572 triangles. The Surface Deviation shows 0.004406 mm which you would think would be fine enough. (This is for a 12 inch diameter cylinder with a 4 inch height and a wall thickness of 2.4 mm. The numbers vary depending on dimensions.) However you can see this level of deviation in the surface of common FDM type 3D prints. For those who are Metric impaired this is a deviation of 0.00017 inches or about 2 ten thousandths. The situation I present is the best case for an FDM type 3D printer and for just about anything else this level of refinement would be overkill. This is partially some arbitrary limitation in Fusion 360 but it is more a characteristic of using STL files. For my purposes being able to specify a surface deviation two or three times more precise would probably eliminate the visible artifacts. Increasing the refinement to the maximum possible will slow Fusion 360 when it computes the STL file and it will slow the slicer down as well but other than this there is no downside to using maximum refinement.
