You can use the YouTube Options extension for Chrome:
Update (2014-04-12): the YouTube Options extension for Chrome now costs $2:
"YouTube Options" cost $2.00 per month, below is a copy of the user agreement one has to confirm prior to using it.
This extension is based on a paid subscription model. It costs a flat
fee of $1.99 per month, with half of all proceeds going to non-profit
organizations. Why the monthly fee? YTO is a fully supported,
ever-evolving product and half of the subscription fee goes toward
sustaining the level of effort needed to develop quality software. But
we’re not just trying to make our product better — we’re trying to
make the world better, too. The other half of the subscription fee is
donated to causes we believe in. Visit the SPOI site to learn about
the charities we are supporting this year. For more information on the
ways you can customize your YouTube experience, explore the options
here, or get the full list of more than 120 configuration options on
the YouTube Options site. Until you purchase a subscription license
using PayPal, YouTube Option’s functionality will be disabled.
Which means I won't use it anymore.