Firefox/SeaMonkey does not display the size of file which is about to be downloaded, not until you click the Save File button and open the download manager to have a look for yourself.
Show-File-Size2 is a Firefox/SeaMonkey add-on that tweaks the default download dialog such that the size of file being downloaded is displayed on it. Refer to the Screen Shots below how the file size is displayed.
Screen Shots
