Using the IDE you can generate a size report.
- Make sure nothing is selected on stage.
- In the properties panel next to Profile : Default click Edit
- At the very bottom and the heading Advanced there is a checkbox labeled Generate size report.
When you run your file the size report will be displayed in the output panel and I think a log file is also saved.
I don't know if this will give you the information you want though.
You could also have a look at this article which has a good method for preloading.
http://www.8bitrocket.com/2008/4/22/Tutorial-Preloading-Actionscript-3-AS3-Games-in-Flash-CS3/
Basically you just put all your assets on the second frame so that the preloader will load first.