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: https://stackoverflow.com/questions/4641272/standard-size-for-html-newsletter-template
And the general NL guidelines from that answer here.