Expanding on samanthasquared's answer, you could set the default to 600-640 wide and media-query it UP to 960.
Otherwise here is an example of a fluid (elastic) implementation in email. You can also set a max-width on a fluid design so that it fits 100% on screens smaller than X width only, and looks like a regular email when the screen is big enough.
The following clients do not support media queries:
- Gmail
- Outlook.com
Yahoo**- Lotus Notes 6.5,7,8
- Android 2.1 and older
- MS Windows Mobile 6.1,7,8
- MS Surface
- MS Outlook Exchange (Android)
- Blackberry OS5
Typically 600 is the "sweet spot" because when people read their email, the preview window is usually about that big taking into account the different page elements of each email client.
Update: Here are 2 resources for Media Query support in email clients:
http://stylecampaign.com/blog/2012/10/responsive-email-support/ http://www.campaignmonitor.com/guides/mobile/
** As of March 2015 Yahoo! Mail Now Supports Media Queries (post in emailonacid.com)