(In Yosemite -- can't remember if this works in previous releases) try...
In Finder, click on All My Files in the sidebar, then select View > Arrange > Size (ctl-cmd-6), then View > As List (cmd-2).
It should show you all your files, sorted by size and grouped into bands by size (e.g. 100MB to 1GB).
If the individual file sizes aren't being shown, then right-click in the bar above the list (e.g. on 'From 100MB to 1GB') and select Size from the drop-down list.
Hope that helps...