Does size really matter?
When it comes to emails, the simple answer is yes, size does matter.
Email clients, whether it be Outlook or a web based client such as Hotmail or Yahoo, have limits on how much. Adverts or folders on the side reduce the amount of real estate the email gets without having to use the horizontal scroll as can the size of peoples monitors. Keeping an email to a maximum width of 600px ensures that in the majority of browsers can view the whole width of the email at all times without scrolling.
Email height depends solely on the amount of content within the email. Where possible, keep emails short and to the point. Being able to keep someone's attention span is a very hard task and becomes even less likely when they have to keep scrolling down.