Welcome to Construct 2!
To the left you'll find Layout Properties, on layout properties you'll found Project Properties. Hit on "View"and you'll find Window Size. You can change it just like Layout size.
I had the same exact question when I started using Construct 2. lol. Be sure to read the manual, try the beginner tutorial and reach out to all the resources this site has to offer. Of course the community is here to help you as well.