Oh that's weird. There should be maximize button on the title bar and also you can stretch the window in any size you want. Also, you can toggle fullscreen with F or Alt + Enter. I assume you have a 4K monitor? I did this because I thought if someone prefers playing in a window rather than fullscreen, this was only solution I came up with in Clickteam Fusion.
Thanks for playing! Yeah I made it a bit easy so people can see everything the game has to offer quickly rather than waste their time replaying same sections again and again.