The level should be as big as your game demands it to be.
I don't think there is such a thing like an average or a golden rule. It all depends on the game your making. There is a limit in Construct 2 for how large your layout can be, but in the end it's all about what you put in your layout. If your game will scroll, if your levels are static or whatever.
What I mean to say is that you shouldn't limit yourself unless it's part of the design. For instance if you make a single screen puzzle platformer, the static screen size could serve for creative solutions.
Bigger doesn't always mean better. A level could be boring if it's too stretched out. Just start planning and building your game and test if it's fun. You can always expand.