So, we are in fact talking about several 'specs' in all cases of designing gizmos, of any kind.

Yeah, I was talking about the specs for the 555 (the datasheet).

Lets say you want a particular frequency and you decide on resistors of 10k, and 1k, and a capacitance of 0.1uF. You (in a perfect world) get the same result if you multiply the resistor values by some constant and divide the capacitor by the same constant.

This means that 100k, 10k, and 0.01uF would be the same, as would 1k, 100R, and 1uF.

However you can't take that too far. 1M, 100k, and 0.001uF might not quite behave the same due to leakage and input currents.

Likewise, 100R, 10R, and 10uF would also behave differently because of limitations in the discharge path (indeed, going too far along this path will damage the 555).

So once you use some calculator to determine the resistor values (or go by the formulae in the datasheet) you need to look further into the datasheet to determine if these values are in an acceptable range. Keeping the resistors between a couple of hundred ohms and a couple of hundred k is a reasonable rule of thumb.