I love playing full screen window on all my games because i have the ability to alt tab to all my other programs with ease. But with maple story, even though its not full screen but a very small window which is hard to squint your eyes to see, i would like to be able to have it expand bigger. Hell yeah
The client would have to be rewritten and since devs don't have a source code for the client, the answer to your question is no. You can try to use window resizer, I remember seeing a guide on the forums. Downside while using the said program is that the quality of the game will be shit since it is stretching images. Edit: After playing around with shortcut settings, I found a way to start the game in full-screen windowed mode. Right-click on the shortcut on the desktop. Choose properties->Shortcut. Find the row where it says "Run" and choose "Maximised". This way windows will force it to start in Fullscreen windowed mode. This method will also stretch the image and make the game look horrible. Any attempts to Alt+Enter will break this functionality and you have to restart the client to gain full-screen windowed mode again.
I've been using the application 'ShiftWindow' which scales your window to your liking. You can make the game take up the full screen in windowed mode without changing the resolution. You can decide to keep the 1.33 aspect ratio or stretch it to fit your monitor.
Windows users also have the Magnifier tool at their disposal which allows for zooming in and out of the screen, making the client look bigger. It loses [minimal] graphic quality but still looks just fine. This client starts the game full screen: https://mega.nz/#!mwJBRD4J!YU64ByH9zda9jHwvPRkh5qtOgGHzVosCmnfqvcLgfng