Need help on something

[NeoPaladin]

So I downloaded a modpack called "Vanilla-Marshmallow" 

When I try to play it, Mc loads fine but the screen is compressed to the bottom left corner of the screen.

 

I already saw something about a Java argument to fix it, but i still have no idea where exactly to put it. 

[nickrcole]

I need more information. What Java argument,

more details on what happens with the window please.

Also Tech Specs and mod List.