Optimum mesh sizing size

That is indeed why I did this sensitivity analysis with ONE simple cylinder (so not a ‘very complex study’, I hink).
And with or without porous media (of this simple cylinder) the velocity results are non-converging for smaller mesh size. So what to do? Or how to interpret? Or is the Communtity plan already too limited for this very simple model?
But I get the feel this is still an art.