Zooming in:
Windows
and Linux : press Strg und + (plus)
MAC
OS X: press cmd and + (plus) key
Zooming out:
Windows and Linux
: press Strg and - (minus)
MAC OS X press cmd and - (minus) key
Return to the original size:
Windows
and Linux : press Strg and 0 (zero)
MAC
OS X: press cmd and + 0 (zero) key