keyboard shortcut to zoom

Probably an assistive setting; you can see what's enabled and what's not, and what shortcuts are configured using  > System Preferences > Accessibility > Zoom. That's from macOS Sierra (10.12) and High Sierra (10.13) and recent releases. I'm not certain if the path to those settings is the same for the OS X Mountain Lion (10.8) or OS X 10.6 releases.

If they've been enabled, ⌥⌘8 is a zoom toggle, and ⌥⌘= and ⌥⌘- are zoom in and zoom out, respectively. Alternatively if it's been enabled, the ⌘ command key and a mouse or trackpad gesture can also be used. The ⌘ and two fingers up or down on the trackpad for zoom in and zoom out, for instance.

⌥⌘8 is a keyboard chord comprised of the command key, the option or alt key, and the 8 key.

If you're in the Safari web browser, ⌘+ and ⌘- can be used to zoom (just) the Safari page rendering.

Related:

Mac accessibility shortcuts - Apple Support

Mac keyboard shortcuts - Apple Support

If the whole display has changed size, then it's possible that the display settings have been changed.

That's independent of the accessibility.

 > System Preferences > Displays has the controls for the available display resolution(s).

What's available for resolutions depends on the macOS version, the Mac, and the particular display in use, and sometimes how the display is connected to the Mac.

Have you tried a restart?

You are welcome.