Resolution depends on three factors (i) efficiency N (ii) alpha or selectivity $\alpha$ and (iii) retention factor k, which the same as the capacity factor in older texts.
Guess which factor is dependent on the column length? It is only the efficiency. If you recall the resolution equation:
you need the desired resolution, say 1.5, you already know $\alpha$ and ks from the experimental retention times. All you need is to solve N. You would also know that N scales linearly with the column length. With these hints you can estimate the necessary column length.
Other ways to increase resolution, if it were a research problem: You would use a smaller thermal gradient, change temperatures, and lastly change the column length.
In the real world, people would use brute force, i.e. they would change the column, because it's the selectivity which affects the resolution the most. For example, pharmaceutical companies test about 8-16 columns at once (HPLC). The column which separates a given sample with their fixed mobile phases is chosen for further optimization.