Fractional goes decimal at very small sizes, which are expressed as "thous" or "mils" - i.e. thousandths of an inch. 0.5 µm (microns) would be about 0.02 thous (or 0.2 ten-thousands of an inch).
Metric more or less rules at the smallest sizes, as it also has smaller measures than micron. There is the nanometer (nm) which is a thousandth of a micron. So 0.5 microns would be 500 nm. Below that comes the picometer...