I'm wondering because I have seen a lot of different answers. Ultimately I would think 500-600px would be fine. Especially if you are building to match standards for Hotmail, Gmail, Outlook, Mail.app, etc.
But I would like to hear others' opinion on the matter. Is there a set size perfect for developing e-mail newsletters? This is talking about building the template with HTML tables.
IMO you shouldn't set the width. I, like many others, tend to view my email on my phone. The population of users with smartphones is growing; so is the population of users that also use their phone to check email.
You never know what resolution a user is at. If you want them to actually read your info, and not just delete it since it causes annoying horizontal scrolling, it would be a good idea to avoid a set width if at all possible.