Maybe this is an old topic (but I didn't find anything on google or this forum):
Would just be nice if a copy-paste between PS and FL was an easier thing.
I know there can also be a pixel-aspect somewhere (like used for video productions and wide-screen), but I didn't find settings for that anywhere.
I still use Flash Professional CS5.0
I'm on windows7 with CS5.5 and transparency and size are copied over just fine. You answered what would be my first question, 72dpi. How about the image mode. Are you using RGB 8bit?
I've had lots of issues with the OS not handling transparency very well also, such right-click on an image on a website that you know is a PNG and has transparency, then paste it into photoshop, the transparency isn't saved.
The route that always works (but is a PITA) is of course to save it as a PNG with transparency and just import it.