Mark's right, Gary, no need to bother with the shell when pure appleScript and the finder can handle this one, with a bit of recursion.
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property foundFolders : {}
set AppleScript's text item delimiters to tab
GetBigFolders(path to startup items folder)
set AppleScript's text item delimiters to return
display dialog foundFolders as text
on GetBigFolders(startingPoint)
tell application "Finder"
set bigFolders to (every folder of startingPoint whose physical size is greater than 3000)
repeat with thisFolder in bigFolders
set the end of foundFolders to {thisFolder as text, size of thisFolder as real} as text
my GetBigFolders(thisFolder)
end repeat
end tell
end GetBigFolders
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