The dimensions are variable.
User Joojaa is correct when they say that "You can no way know what size or shape the recipients screen is going to be."
You cannot set any explicit value in your CSS, because if you want to consistently support all browsers, you have no way of knowing what size it's going to be.
I'm going to give you one method, using just HTML and CSS. I have briefly tested a basic version in Chrome and IE. It's fine back to IE7, and then the vertical alignment drops out, but the single page with no scrolling remains intact as far back as IE5(!).
Here's an image of the result (click on the image to go to a working example on jsFiddle):
In case my example link goes dead, here's the code for my mockup:
“I'm part of the 21<sup>st</sup> century now.<br> I have an online portfolio!”
<a href="#">Click here to see some of my awesome work!</a>
font-family: 'Open Sans', sans-serif;
border-top:4px ridge #4c4ca3;
border-left:4px ridge #4c4ca3;
border-bottom:4px groove #4c4ca3;
border-right:4px groove #4c4ca3;
Good luck with your project.