The resolution of the ELF corresponds to the grid mesh cut-off used in the calculation, so if you increase that, you get more details in the picture.
However, there is a trick to getting a picture with higher resolution without recalculating anything:
conf = nlread("file.nc", BulkConfiguration)[-1]
conf.calculator().numericalAccuracyParameters()._NumericalAccuracyParameters__grid_mesh_cutoff = 500*Rydberg
elf = ElectronLocalizationFunction(conf)
nlsave("elf.nc", elf)
What this does, is it tricks the program into thinking you actually used a higher cut-off in the calculation. Of course, it's cheating in a sense, or rather, call it an interpolation.