There are many rules of PCB design,regarding the trace width and spacing,we can refer to IPC and UL standards.
We can look up for the guidance in minimum spacing requirements for PCB board. For example,a PCB board runs on 24V, the spacing requirements are 4mils or less,so you must consider the operating environment.
RCYPCB suggests to define the the trace width and spacing with the lower limit at 5mil,otherwise you have special purpose. If you define the trace width and spacing less than 4mil,there will be a lot of manufacturers have no capabilities to build.
Our best capabilities are 3/3mil for outer layer and inner layer trace width and spacing at Hoz copper foil.