NY Times has an interesting infographic that quantifies the blue wave:
“Over all, 2018’s shift to the left was smaller than the one in 2006, the last time the Democrats flipped the House. And it was half the size of the most recent Republican wave in 2010 when districts shifted more than 19 points to the right.”
https://www.nytimes.com/interactive/2018/11/07/us/politics/how-democrats-took-the-house.html
That's a bit misleading as you're comparing a massive blue wave in 2008 to a red wave in 2010 while comparing a basically even contest in 2016 to 2018.
The NYT has a more quantitative evaluation here which shows just how impressive the Democratic wave was.
https://www.nytimes.com/2018/11/07/upshot/2018-midterms-blue-wave-democrats.html