I fought a bigger taller fighter in my first fight. He had a good reach advantage and unfortunately for me he had pretty good takedown defense. It was a long night for I'll say that.
Work your game. If thats boxing then use your explosiveness to close the distance and nullify his reach. Dont stay outside and get picked apart. If you clinch you sound like you know what to do. Try to get a solid side control a beat him up there.
Before your fight, train off your back. Do some sub escapes. Basically train out of your worst case scenario. You need a plan in case things dont go your way. Do that and you'll feel more confident that you can handle whatever happens.
Good luck man, let us know how it goes!