500-699 CFM microwave

https://www.bestbuy.com/site/searchpage.jsp?_dyncharset=UTF-8&browsedCategory=pcmcat748301803023&id=pcat17071&iht=n&ks=960&list=y&qp=maxairflowrange_facet%3DMaximum%20Air%20Flow~500%20-%20699%20CFM&sc=Global&st=categoryid%24pcmcat748301803023&type=page&usc=All%20Categories

400 CFM is enough for 12x16.   But you can get higher power microwave.  

Also think about microwave hood extension https://microvisorhood.com/