I think you need to set it to Letterbox scale on the configuration settins, that way it will remain its own aspect ratio and remain centered on the browser though it will scale nicely =)
(letterbox scale integer does the same but won't let it scale BIGGER than the layout size)nfortunately it's not this way in Construct Classic, which is what the question concerned.
It's very easy to do in CC as well, though only for unscrolling layers. You simply just use the system condition Set Display size and the game will be rendered in the center of the letterbox after which you can set the game to run on fullscreen which in turn will give you the proper aspect ratio for the game (assuming the aspect ratio matches that of your display's, but you knew that already).
With scrolling content however the most straightforward solution is to create an unscrolling layer and create the letterboxing yourself. You'll also have to make use of Unbounded scrolling to make it possible to scroll outside the layout bounds, which is necessary for the object positions to appear correctly. You'll however be quick to notice that with Unbounded scrolling you'll have to account for any space outside the actual layout bounds, because the scrolling will reach these areas.
Here, I baked something similar to what you'll need. CC r2 cap: