When sending email blasts you want to keep the file size as small as possible. This will ensure that the email loads fast for your prospect. To do this you will want to make use of clean code, optimize your images and use inline CSS correctly to keep the file size down. The design of your email blast should be no wider than 600 pixels, so that the email can be clearly read in smaller screens or in the outlook preview panel.
The best dimensions for an email blast is less than 500-600 pixels wide by 800-1000 pixels tall. The reason you want to stay within those dimensions when sending an email blast is that most of your recipients will be on a screen 1024-1280 pixel wide. If they are using Outlook, the preview pane ends up being around 690 pixels wide. By properly formatting your email blast design, you will ensure readability.