How specific can I get setting layer heights?
Layer hight can be theoretically anything as long as it fits into these ranges:


it needs to be at least one step for the Z-Axis motor to be physically possible
it should be at max 3/4th of your nozzle diameter to create an adhesion surface


In praxis, the lowest setting for layer height due to physical limitations of the extrusion systems is around 0.05 mm. 

Also, not any number is possible, it is dependant on the Z-axis system: since one or one-half step is the smallest rotation that the stepper can achieve, the raise that belongs to this partial rotation is how much one and the next layer height can be together. The step limitation usually is of no concern though, unless one has a very steep lead screw.

As a rule of thumb: doubling layer thickness results in almost half the print time. 0.1, 0.2 and 0.3 mm are common because they allow easy checking the printed accuracy in Z-axis with calipers.