"When studying any macroscopic system with a very large number of degrees of freedom, invariably make an approximation and simulate a smaller and/or discretized model system. This introduces systematic errors called finite size effects. Have to understand these, and be able to extrapolate to an infinite system, usually by doing a number of simulations at different system sizes."
But I am still a bit in the dark side. How for example, If I have a set of data from a system of for example 5 degrees of freedom, if I analyse and plot the data, how would I know that I should look at a larger system? How could the plot tell me that I probably have a finite size effect problem?