Finding out how much space is used by a SQL table is pretty easy. I've been using a script I found back in ~2008.
http://www.sqlteam.com/forums/topic.asp?TOPIC_ID=61762
I was amazed to see that the article still comes up at the top of the list when I google for the latest and greatest "how-to" on the subject.
Anyway, the way the script works doesn't look like it can be changed to work on just part of the table.
For instance, I've got a table, let's call it, Error_log. There's errors in there going back to 2015. I want to see how much of that table's 9Gb of space used is in the rows where the err_log_add_date is before 1/1/2017. Is there a simple way of doing that?