Currently you can only zoom in/out via either the Zoom toolbar/the statusbar or via Ctrl+Mousewheel.
It would be good to have a keyboard shortcut to do the same.
Ctrl++ and Ctrl+- are used in other applications, however, they can't be used, as these shortcuts are taken in Calc.
So, Alt++ and Alt+- seem like a good idea.
i will see if it can be done.
See Bug 45705 for workaround
Oh... this is a duplicate then.
*** This bug has been marked as a duplicate of bug 45705 ***