Different results of S11 parameter for different mesh element sizes

Hi,

I have achieved two very different S11 parameter plots for the same patch antenna simulation at "extremelly coars" and "normal" element sizes of mesh. I attached these two results as image files to this question. "normal" one is more similar to the measurement results. But how can I be sure which element size is enough for me. Because, the finer element sizes are very heavy for my computer.

Thank you