Here are some examples of Flash movies being placed into HTML (below). Notice that in all of these examples the width and height are set both in the <object> and <embed>.
It's important to make sure that the Flash content you create will be viewable by your audience. Whether your audience can view your Flash content is heavily dependent upon which Flash Player they have installed on their machine. Here's a link to the Adobe Flash Player Version Penetration Statistics.
Fortunately, Flash has a way to publish with Flash Player detection using HTML and JavaScript. A step-by-step explanation of Flash Player Detection is available on Adobe's LiveDocs.
A much better Flash detection has been authored outside of Adobe, however. It's called SWFObject by Geoff Stearns. SWFObject is written with JavaScript and uses cleaner code. It also allows for 100% scaling while validating as XHTML. I would highly recommend using this one over Flash's built-in publishing methods.
Here's a short list of some of the Firefox extensions I use when developing: