The schematic appears to use mosfets as the active components. I'm not sure why the units are micrometers (which is a physical parameter, not an electronic specification). Is there a current path through the inductor at M1? My knowledge is incomplete of course .
Anyway, as to what value to use for a current reference resistor...
It is not necessarily an automatic choice. It must be determined through experimentation. There are many variables which must be factored into the equation.
Example... The resistor network that produces 5mA reference in the schematic, may end up producing 5.5mA in real life if one resistor is 10 percent off. (This may or may not be a problem in this particular application.)
Or the reference network may produce 6mA if you raise the supply voltage by 20 percent.
With real components, if it is important to have a particular value as a current reference, then it will require direct checking of values, and adjusting to suit the requirements. At least until experience shows you what will work, and what won't work, without a need to check.