The best way to get better separations is to change the selectivity of the stationary phase.
True in general, but for fixed gases (as in this case) there are only a limited number of options.
To address the original question: to a good first approximation, if you double the column length you will improve resolution by 40% (doubling the length doubles the plate number, and resolution is a function of the square root of the plate number). However, the retention times will approximately double, and the pressure required to maintain your flow rate will also increase. Assuming you are using a molecular sieve column, the selectivity (relative spacing of the peaks) should remain about the same.