[Canadian]

A longer overflow (read: larger linear dimension of the overflow) will allow greater velocity of surface skimming and more efficient removal of surface-bound proteins. Instead of pulling non-protein laden water from below the surface with a small overflow you maximize the removal from the surface with a longer overflow. Ultimately the flow rate could theoretically be the same for both the shorter and longer overflow but the longer one pulls more preferentially from the surface as opposed to the shorter one pulling water from under the surface. Therefore the shorter one could have the same flow rate but actually take considerably longer to remove the surface-bound proteins rendering the shorter one being considerably less efficient and unnecessarily wasting energy consumed by the return pump (assuming you're trying to match your overflow rate to your protein skimmer flow rate to maximize efficiency and not using your return pump to add flow to the tank).

This the basic principle behind the "Calfo style overflow" which greatly enhances surface skimming. Take a look at the link in my previous post.