First up, make a copy of your project, then resize it.
Go to Project menu > Resize project and make it a size that will fit in the browser window without scrolling.
You want to do the resize in one step, as if you resize the same project a number of times the quality will degrade. Which is why its good to experiment with the size on the copy, so your original project stays pristine.
Hope this makes sense, and is what you were after.