It is not by any account the largest museum /site,...

It is not by any account the largest museum /site, but don't be deceived. In addition to the life of MLK, the museum gives good insight into the feelings at the time. It took about 2 hours to go through the museum (could be done quicker). Surrounding sights will take at least another 30 minutes to see. Where MLK grew up, typical house of the time, as well as MLK memorial. These are definitely worth it.
There was a large parking lot across the street. It was free.