I have to design this e-mail newsletters.
Some with just simple holiday cards and some more complex with offers and other stuff(images, text content etc. etc...) I am wondering if there is a safe standard width to work at ?
The majority of the newsletters have a defined width of around 620 total pixels.
I usually use 600px, but anything below 620 is safe. Most email clients and web-based email providers don’t use the full width of the screen to display an email message. Whether it’s ads on Gmail, a menu in Hotmail or your Inbox in Outlook, a chunk of screen is often already being used (source).
Similar question in stackoverflow: http://stackoverflow.com/questions/4641272/standard-size-for-html-newsletter-template
And the general NL guidelines from that answer here.
There is no standard width because you have to take into account mobile devices. It should be fluid. A good HTML E-Mail framework is http://htmlemailboilerplate.com/
Although I asked the question, it's been a while, since then I've improved my knowledge on the topic:
After my research I would say the best way to design newsletters is to use Responsive Design instead of relying on one single standard width size...
It's better to make your design responsive because nowadays people read their email on a wide range of devices(SmartPhones, Pads, Desktop PC etc)
more info about Responsive Design for newsletters in this article: http://designmodo.com/responsive-email/
The width is what you need to worry about, as for the height, need not to worry, the user can scroll his way through your content, since the apparition of Facebook, Twitter and SmartPhones, the user is much more used to scroll(vertical scroll that is, NOT horizontal) his way to see the content, but of course, keep in mind that he might not do that, so place the important stuff as high as posibile in your design.
This would be the very basic stuff that I can add to the above answers, but again, just to be sure, do your own research, google stuff you wanna know, and you'll end up creating a neat newsletter with great response from your audience.
Footnote: I will not mark any answer as "The Accepted Answer" yet because this topic is worth researching for before getting to work.