Zooming in and out

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