1)The square made of intermittent lines on the canvas, can it be bigger? I am making a scrolling shooter and that area seems to be too small to fit player and enemies without sacrificing details. Just wanted to know if it can be done and if so, how?
Are you talking about the layout ?
You can set the layout size in the layout properties.
Not sure it is what you're talking about though.
2)If I plan to make stages on a big scope (giant scrolling stages as in famous platformer games) can it be done without making too many layouts? Does construct support this? I mean one giant hi-res image where I move the player through.
A single big image is bad design as it just consume vram uselessly.
Have a look at the how do I FAQ and check the section "Scrolling/Camera" you'll have multiple examples of how to perform a scrolling with C2.
An yes, it allows you to make large levels in the end, but you have to be smart about the way you set them up.