Yes, as bogu stated, you can specify the width. Now, be careful. Even though it defines the width of 750px, content can still overflow past the edge. The way to combat that is good CSS:
width: 750px;
overflow: auto;
With overflow set to auto the div won't be larger than 750px, and if content wants to be 800px wide, a scroll-bar at the bottom of the div will show up for the user to scroll. It's annoying, but it works. Helps guarantee with width will be the same.
You might also use a "wrapper" to wrap all the content in the site. The wrapper would be a basic <div></div> but with a specified with and overflow handeling to give you what you want. You can see this in action at patternet.com.
Honestly, if you're going to be specifiying the width of the page, just create the page in straight CSS (or convert it) as it will make things much easier, and later, when you say "Hey, I don't like 750px wide, I want 800px wide", it's a matter of changing one file (the CSS file) and the changes take place site-wide.....
