After awhile of painful looking graphics, I installed the Widescreen Mod but realized my monitor, at 1366x768, is a bit too big for the game, making inns and houses painfully tiny. I'm new to messing around with resolutions and was hoping for some suggestions on what would be good, but not stretched looking at all.

I always go with 800*600 (sometimes with 1024x768, but even if it is bigger and uglier, I prefer the 800*600 resolution) + this GUI mod to avoid black zones among the GUI bars.

You can try 1024x600 if your card supports this resolution, because it's about the same aspect ratio as 1366x768. It works on my laptop.

I already installed the whole BG trilogy mod, which makes reinstalling the widescreen mod with a different resolution difficult, seeing as it adjusts the game window to that resolution, but leaves the screen at the previous resolution and just has blackness around the box. But, the resolution size looked like it would have been perfect if it hadn't been for the black bars.

Baldur's Gate II (and therefore your Baldur's Gate I because of the mod) supports somewhat higher resolutiones per se. Even if you can achieve higher resolutiones with the mods, I highly recommend using the ones "setup-able" via the "config.exe" in the main directory. For BG 2, I usually go with 1024*768 or 1280* something, and never had an issue.

I'm playing Baldur's Gate 2 at 2560x1440 currently and i really like it :)
Didn't even install a larger font.
This high res just makes the view circle appear a little bit "unsmooth" I wouldn't recommend this way. I got the problem that, while using unsupported resolutions from the game configuration, several items in my inventory will always let the game crash on being inspected with right mouse button click (one item in the tutorial, another in the room with golem next to your cage in Irenicus Dungeon)
I can't remember that ever happend back in 2001 when I first played the game, but I'm also not sure I ever used a resolution above 800x600.

I have a 17 inch monitor, so the size was originally too small feeling, but after an hour of playing it I actually got used to it. The larger font is nice, but it does have a few downsides like the text not always fitting in the lines. Either way, the user-made guide I found on GOG for installing the BG Trilogy mod was awesome.