How to restore the default dimensions of the Force Quit dialog

Holding control only shows those dialogs if a script is saved as an application. Couldn't you just save the script as scpt or applescript?

I'm afraid that there really isn't a way to do it. I've been running into the same issue. The effective resolution at 1440x900 is actually 2880x1800 pixelwise- so the reason that you see crisp text is because you are looking at an image with 2880 x1800 resolution on a capable display. Because scaling down usually involves smoothing, the images will look terrible in email /on other peoples computers.
One solution is to download a program such as SwitchResX that allows access to "non-HDPI" resolutions. This would allow you to set the display to a true 1440x900, take the screen shot, and switch it back. Best of luck!