[Phanx]

Well, first, go check your game settings and make sure it's actually running at the new resolution. I don't know that it will pick up on that change automatically. It may still be running at 1680x1050 and just getting stretched to fit the larger physical monitor size.

Second, check your "Use UI Scale" setting. Ideally, it should be unchecked so the UI will scale itself so that 1px in the UI = 1px on your monitor, resulting in the best possible appearance (eg. no "fuzzy pixel" problems).

Finally, you should just be able to adjust the sizes of individual elements of your UI. I don't know about the specific buff and minimap addons you mentioned, as I've never heard of them, but pretty much any addon with any options at all should have an option to change its size. As a last resort you can either make your own "addon" to run SetScale on specific frames, or enable the "Use UI Scale" setting and lower the scaling value to make everything smaller across the board, though you may notice "fuzzy pixel" effects as each pixel in the UI will no longer match up to a physical pixel on your monitor.

If you want to look for replacement addons, click the search box in the top right corner of the page and type in "buff" or "minimap" -- there are plenty of both to choose from!