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I should add that the very lowest option (320x480) is the only one where the resulting window actually fits inside my screen. However, choosing that same option repeatedly in the same window keeps re-sizing the window slightly, which reduces my confidence that anything is working correctly (including whether the width:height ratio is even correct).
Hi Ionut - I just answered your other reply to the same problem I posted as a separate thread - see my answers there. But I want to add that even when I try the 640x960 resolution, the window is STILL so big that its bottom part is beyond my screen, so I cannot see what actual size it is. Clearly something is not working right when the re-sized window doesn't fit on my screen at a lower resolution but fits on my screen at the very highest resolution (1920x1080).
Hi - I'm not sure what window vs. viewport means. I'm just using this add-on's icon in the top right, the way it came.

I'm trying to get it to 1024x768 size. I cannot see the actual size I get because the resulting window is so huge it stretches beyond the screen on the bottom. So clearly it's not actually 1024x768 size, which would be a lot smaller than my screen (and which it used to be until the add-on got broken a few weeks ago). The aspect ratio of this huge window may very well be correct, but that doesn't help me any.

Not sure what "window.devicePixelRatio value" means.

I've tried this on two different computers now (one home, one work). Both have identical specs:
OS = Windows 7, screen resolution = 1920x1080, Chrome version is 35.0.1916.114 m.

I second this - the newest version of Window Resizer does something weird where the resulting window is freaking huge. It has the correct aspect ratio, as far as I can tell, but it's many times the size of the actual display you're trying to emulate. This basically makes it unusable.