It depends on the maximum current you need to flow through the circuit,
For the same RC value,
with smaller R, bigger C: you will have larger maximum current and more energy stored in the capacitor. YOu might need this for C to act as another power device.
with larger R,smaller C: you will have smaller maximum current and less energy stored in the capacitor.you might need this for using RC as timing device.

[quote]m delaying the continous square wave signal through an RC circuit ...[/quote]

I do not understand why square wave can be delayed with RC circuit? The wave form will be changed. It is not a delayed waveform.

Please describe in detail what is your application (and specification). 20 ns is a very short time. If you just need to delay signal, you should use a coaxial cable with length $L=3\cdot10^8 \times 2\cdot10^{-9}=0.6 \,\rm{m}$.