Demetrious Johnson: I Can Beat Brock Lesnar, Here's How

[[tmz:video id="0_yn31kvh8"]] UFC flyweight champ Demetrious Johnson says size actually doesn't matter ... telling TMZ Sports he thinks he could actually beat Brock Lesnar, and he has plan on how to get it done. FYI -- DJ is 5'3", 125 lbs ...…