Been occasionally playing BG2 with no issues for over a year now but today when I started it up, the game is a smallish box in the middle of the screen surrounded by black. As far as I can tell, all the settings still say it's set to "full screen," and yet it's only about 2/3 of the screen. It's possible this is due to my changing my computer screen resolution a couple weeks ago to play a very old cd game, but I changed the resolution right back.

When I take a screenshot it crops out the black so I took a screenshot and sort of recreated what it looks like. Any suggestions are appreciated!

It's probably a desktop setting. Open your video card control panel and go to the desktop size and position section. Make sure that scaling is set to aspect ratio, and choose your preferred option for scaling. Scaling on GPU will present you with a 'letterbox' effect (black side borders) if you have a widescreen monitor, while scaling on monitor will fill the screen but the image won't be the original aspect ratio.

My graphics card is an Intel HD Graphics 4000. I don't see a section for desktop size and position. I tried looking up where to change scaling but I guess I'm not as computer savvy as I thought I was. I don't even see an option to change scaling. My options menu looks like this except the resolution is 1366x768.
https://communities.intel.com/servlet/JiveServlet/downloadImage/2-205030-233145/Snapshot+1+%2805-09-2013+22-39%29.png

This is an issue with the Intel drivers. Intel are aware of the issue, though why it should even be an issue in this day and age I have no idea. The best option currently is to launch the game while in your previous desktop screen resolution. You may also try to change from windowed to full screen while in-game by pressing the ALT+ENTER keys.

There is also another option that you may try with BG2: run the BGConfig.exe program and disable OpenGL acceleration, then run DDrawFix.exe and choose to Force DirectDraw Emulation.

I have the same issue too, but as Hickory says disabling OpenGL acceleration solves this (you loose smooth fog and some other things though, nothing very important though).

As to the scaling option, on IntelHD (and Optimus), you have to lower your resolution, then look at the scaling setting - you will be able to change it. Hit 'Apply', then revert back to your native resolution, it won't show anything but "full screen" scaling, but previous setting will be saved for lower resolutions. Resolutions in the same format (16:9 etc.) as your native resolution will still be displayed full screen though.
From the screenshot you've posted, I think your driver is outdated (my IntelHD panel looks completely different, the blue one has been changed months ago).

Sadly, on Baldur's Gate 2 it doesn't change anything, I've tried several settings, and OpenGL display (for this game) is always displayed as if in "centered" scaling. This is odd.

If you want to keep OpenGL activated, you may consider installing this mod:

http://www.shsforums.net/topic/52452-bigger-fonts-mod-for-baldurs-gate-2/

I'm not really into BG2 at the moment (playing Neverwinter Nights) but from what I've seen onscreen, it should at least help with the text being too small, especially in centered scaling.

What I ended up doing was putting my computer back at the previous resolution and changing the scaling settings while in that. I swear things still don't look as good as they did before but I don't know what else to do. Tried to run the widescreen mod and it didn't work. Tried updating my driver but apparently each computer company releases its own version of drivers and I have the most up-to-date version allowed by my computer. Oh well, at least I can still play.