(") Mean inches (') Means feet At no time is there a (') in the post. Yes you are reading it wrong. Had it been 20'1/2" x 21'1/2" than yes it would be 16'x17' but it's 201/2" x 211/2". See my reply to skyrimdog.
This an interesting item. However, the Stars and Bars has a very different design. Here is a good web site to check out: http://www.usflag.org/history/confederatestarsandbars.html