In Windows XP and 2000 you could search based on file size being greater or smaller than [x] Kb, but if you're looking for a file in Windows 7 based on anything other than finding a word in the file name or inside the file itself, the only option I've been able to find is what 7 calls "adding a search filter" such as in the diagram below which is, in my opinion, a wee bit limited.
I've been using the CoMmanD line to do 2/3 of all my file searches in Windows 7. Say you want to find every PDF in all the subfolders without listing every document that happens to have the letters PDF in the filename or anywhere inside the file itself. CMD is a life saver in such circumstances. However there's nothing in the DIR command to limit file results (maybe SP1 of Windows 7 will include a switch like /SIZE>2000MB).
I'm probably just going to end up using WinDirStat, but seriously shouldn't you (still) be able to do this in Windows Explorer?