Yep, I realise that. And unfortunately to the best of my knowledge there is no way of doing this. The flash object does not get a width or a height until the browser rendering engine assignes it when the page is being renderered.
Even if a flash developer gives the move canvas certain dimensions, that does not mean that those will be the sizes that the movie is rendered at.
Hi - Open it in the Flash Player and use something like MW Snap to get a screenshot of the window. Crop out the menu, etc. and the resulting dimensions of that image should be close enough for you to use.