If you use tables and percentages for table width, then you have some control over the proportion, but you are really at the mercy of of the end user's settings in any page you design. If you use layers you can indicate absolute position & size, but again you need to design around what the majority of your audience will be using to view your site. 600 x 800 is still one of the most common screen sizes... but actual viewing area will differ depending on browser they are using and how many tool bars they have open, etc.
In the end, you need to choose sizes and layout that will look best at the most common resolutions, but will dregrade well at all others.
With flash you can specify either percentage of exact size of the movie. You best bet is to play around with the variables and test your page on different resolutions.
Anyway, what you mean by size is a bit vague to me. If you mean you want the flash movie to be the same relative size compared to the monitor, yes, if you mean an exact size regardless of monitor, then no. If you set it to 50% of the screen, it will obviously be smaller on a 15" monitor than a 21". If you set it to say 300 x 400 pixels, then it will appear larger on a screen res of 600 x 800 than it will on a screen res of 1152 x 864.
hope this helps When in doubt, deny all terms and defnitions.