A simple google search using a cut-and-paste of your question text returned about 2.6 million results.
I found an answer to your question in the first 10 results.
how to find folder size using sql scripts[
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from
http://www.sqlteam.com/forums/topic.asp?TOPIC_ID=156137[
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"You can use xp_cmdshell to call the dir command with the appropriate flags, or maybe a Powershell script, but that's about it. There are no native SQL commands to provide that information."
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As per your additional information, I am not entirely familiar with doing this, but you may need to create a process that will create a running tally of the sum of file sizes and then iterate through each sub folder. This would be a very inefficient and time consuming process if the directory structure gets to any substantial size, so hopefully someone can provide you better info as well.
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