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Although I have not seen that message pop up in my own work before, it seems to be very clear to me in what it is saying. It even tells you what to do -- that is, you need to modify your mesh settings to generate a coarser mesh, or shrink your problem size, or both. Consulting the Comsol help system should help you learn more about generating meshes. There is also a video at https://www.comsol.com/video/building-the-mesh-for-a-model-geometry-in-comsol-multiphysics . If you are new to Comsol Multiphysics, then I am guessing that this is occurring because you are naively attempting to model an electrically-huge volume, likely (but not necessarily) using physics-controlled mesh settings and, if so, that the software is trying to mesh finely enough (just as it should) to track the details of the RF waves. Am I close? Consider posting your .mph file to the forum so others may take a closer look, if you want a more reliable diagnosis.

Very often default meshing works fine. When it doesn't it's usually because the domain size is very large and/ or there are features with dramatically different size.

To get useful advice OP will need to post the (reduced) model.