I currently have thoses kind of problem with my game and to be honest i am quite frustrated. (Not fault of Construct 2)
I have a fully pixelated game and even if letterbox integer scale seem the best solution (as told by Ashley) this is not a good option at all. This will end up unprofessional. Why? because i can simply not go fullscreen without seeing tons of black area (because it didn't go really fullscreen but instead made the windows size fullscreen)! so what the point to go fullscreen then? most desktop game require a fullscreen mode.
So i started to look for another solution... I avoided seams by having an option to scale up the game at 2x when i reach 1920x1080p in screen size. I use the crop mode. Work good except that you must position everything when you reach the 2x upscaling mode. There another problem.. when you upscale 2x (to avoid seams) you ends up with 2x less place on the current screen size. This can have problems with interface which take a specific minimum size. In the end.. you end up with big layout.. that can reach 1920x1080 in screensize and then reducing the performance.
I think the problem lie in the fact that Construct 2 or Node-WebKit doesn't have the option to enable you to change the system resolution to fit the screensize. Such option would be very useful. I don't think it is normal that i have to make a third party app (kind of launcher) to do that job.
I am a bit out of options... anyone ?