I didn't saw any method so i decided to make one.
1 - First you need to download a little program called "Sizer" -> here. Virus scan -> here
2 - Install and launch it. You 'll have a small icon on your taskbar.
3 - Launch maplestory and press "alt enter" or altgr enter" to have it in a windowed mode (resolution : 800 x 600) and you can drag this window.
4 - Now rightClick on the Sizer icon then " Configure Sizer". Here you can put the easyer config for your screen (mine is 1280 x800 so i just put 1024 x 650 ) so i can play i a higher window and use hacks at the same time without to have to press alt tab everytime . just click "add" then put the width and height you want.
That's all.
I didn't know if a tutorial was needed for this but i hope it 'll help some people.