Hello @Odiaz , Good questions.
The only advice I have is to start rather large (relative to your 50km of coastline) so that the model runs very quickly and validates all the other input data quickly. Then you can always re-run with smaller resolution as-needed. Maybe start at 1000 meters? And consider the resolution of the other input data (landmass & habitat layers). If they are very coarse, it would not be appropriate to go too small with the model resolution.
Generally we look at the coastline in GIS and roughly measure any large bays, inlets, or other areas sheltered from the open ocean. The max fetch distance should be roughly equal to the distance across these bays. Please see the User’s Guide for details on how the fetch distance influences the calculations of wind-driven wave exposure. And for details on some intermediate outputs that let you see the fetch rays as they were mapped.
The model is not overly sensitive to this parameter, but choose something larger than the resolution of the DEM, but probably smaller than the “model resolution”. The idea is to capture the average elevation around each shore point, to differentiate points with respect to their elevation above sea level.