So I got the ultimate collection (yay!) and followed the steps to be able to get 1920x1080 and to get rid of the black rectangles under my sims. The problem is, as I said above, that the ui doesn't seem to have quite scaled right with everything else. Buy mode thumbnails are really small, making hard to find things, and in create-a-sim, the box with all the sliders and everything is smack dab in the middle of the sim's face. It's not a game breaker or anything, but it is annoying, so I was wondering if there was any way to alter the ui, either options in that options file I had to edit, or a mod, or some hidden setting, or what.

Windows 7 64bit
Nvidia GeForce GTX 750
Intel i7 2.93 GHz
8 GB of ram

Could you perhaps take a screenshot?

That is actually the case for me to, that the UI is blocking a bit of my sim, I have never used this mod but it might help you with CAS

http://modthesims.info/download.php?t=292745

Run cas in windowed mode. That will fix it.

@ZenGarden: ok, I tried to do take a scrennshot, but it just ended up taking a screenshot of what was behind the game, i.e. this page :/

Do you have a printscreen button on your keyboard?
You can use that and it copies what's on your screen now at your screen's full resolution.

That's what she did, but it just takes a print screen of the desktop instead. Same happens when I try to take one. I forgot if it was a Windows 8.1 problem or what it was.

As Leefish said, Sims either has to be in windowed mode or a screenshot program needs to be used. I've just been too lazy to do anything about it myself.

To get it to stop printing your desktop, you need to disable the visual themes and disable desktop composition. Right click on your sims shortcut icon, whatever that is for UC, select properties, select compatibility, and check the boxes to disable visual themes and desktop composition. That should do it.

I run the game in windowed mode, so when I go into CAS (which, granted, I don't do that often), I change the game's resolution to 800x600, which makes everything fit properly without having to replace the CAS camera, like you'll need to do with the mod that ZenGarden linked to. Which I'm sure would work, too, but in my case I already have a modded CAS camera that I like and don't want to lose it to use that one instead. Then, when I go back to the game, I change back to my much higher widescreen resolution. It works for me but, like I said, I'm not in CAS much at all.

I think this issue has been discussed before, not too long ago. I remember posting a screenshot of my Sims 2 shortcut not having the compatibility tab in a thread where a girl was having the same problem.

Sounds like an insta-fix for those who do though, so I'd call that pretty helpful.