It's always windowed, there's no option to switch from windowed to full screen in the game. How the hell do I play the game in full screen? It's annoying as hell to have it on windowed all the time, can't enjoy the damn game man.

Alright guys I've found the solutiion!
1. Right-click on Launch Giants - Citizen Kabuto and choose Properties.
2. Click the Shortcut Tab.
3. In the Target field, at the end and outside the quotes, add "-window" (without quotes) as shown in the attached image.
4. Click Apply then OK.

Hope that helps! :D

This is actually very strange problem for me.

What you said did help for first time, but when I wanted to play the game again, it was back in windowed mode and I cannot turn in back to fullscreen mode anymore. I have Windows 8.1