I have been needing to make my files about as twice as big to get the same amount of painting space on my screen as I had in CS4. For instance, I am working on a picture that, upon creation, I specified to be 2000 pixels across. However, when I measure the amount of pixels on the screen, I get only 1000 pixels worth of space. So I have to make my files twice as big pixel-wise.

I am using a 15 inch MacBook Pro with the default 2880 x 1800 resolution.
I'm already using "use same aspect as pixels".
I've unchecked "Enable Hi-DPI support".
I'm utterly baffled.
This is embarrassing and frustrating.

How can I ask Krita for 1000 pixels of painting space and get 1000 pixels of painting space?

You're using an Apple computer. Apple computers cheat you: they only give you half the resolution of your screen because of "retina" and that kind of nonsense. Four pixels make one, so that one looks a bit sharper. Your screen is, effectively, 1440*900 pixels, unless you change that in the settings.

I don't have the same problem with CS4. When I create files in CS4, what I see is what I get. When I create files in Krita, what I see is half the size of what I should get.

Hi elizabeth!

Krita has no notion of scaling for screen in macOS, which is something apparently (for the things you mention) is present in CS4.

In Preferences > Screens, you probably have your retiina display resolution to "More space" which basically does not use the 4px to fake 1px, instead it goes 1px to 1px, so the GUI looks much more smaller and give more space. Krita for the moment does not compensate for that screen scaling, if it detects a retina display it will "scale" to show the image as if it was a retina resolution (which now isn't). Im my screen Krita works correctly on my retina in "predefined screen scaling" and shows the image in half resolution when I set to the "More space" setting.

Don't spend to much time on this, I tested Preview.app and it also made other scalings (it shows the image bigger than it is even if 1px to 1px option is set). I made some more testing, and a retina 2560x1600 display is "seen" by preview as a "1230x800" screen. To test this I opened the 2000x2000px image in Preview and the image never fits the window even in 1px by 1px scaling. Firefox does the same thing. When using screen scaling it also compensates for this and shows the image in the exact same size apparent to the real world, which means, even on "More space" neither Firefox nor Preview show the complete width of the image on 1 by 1 previews.

If you create a 2000x2000 px image it will be 2000x2000 pixels, to rpeview it just compensate for the scaling by seting zoom to 200%. Once you open the exported image it will still be 2000x2000 pixels size.

So yeah, Apple computers are weird. We might need to adjust scaling in the same way as Preview or Firefox do to be consistent with the OS, maybe.

So it's something I shouldn't worry about for the time being, then?

Currently my retina display is "default for display".

Yes nothing to worry about, your image is still the same real pixel size as it was before. Only display scaling is off by some reason.

Now im curious in your setup, since it works flawleslly in mine. What version of macOS are you using? is your display connected as a second display?