If f=100Mhz, T=10ns
That mean the clk signal will toggle every 5ns.
So ns resolution should be sufficient.
There is no option to set the default timescale of the Waveform window before opening it (might be version dependent). Once it is opened you can adjust it by doing a right click anywhere in the waveform window and select the desired timescale.
There are also TCL commands.
Hope this is what you want, how to do it from the GUI.
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The 2nd pic, which you uplaoded just a few sec. ago!
Why are you forcing the signal (clock)?
Your clock should be driven from the testbench, which will apprear in the sim waveform.