Just how big is the market for legal pot? A new report by a leading marijuana industry investment and research firm found legal cannabis sales jumped 17%, to $5.4 billion, in 2015 and they will grow by a whopping 25% this year to reach $6.7 billion in total U.S. sales. ArcView Market Research will release its fourth edition of The State of Legal Marijuana Markets report, and it includes the prediction that the legal cannabis market will see a whopping $21.8 billion in total annual sales by 2020.