A 7.2 or so would be good , I wouldn't go much smaller than that. Unfortunately it probably wont fit your mast. When buying a bigger sail make sure it matches the mast curve bend of your new mast. There are charts to get that right. Dont forget your boom , is it long enough for the bigger sail ? and its also not a great idea to use the boom on max extension. It helps to have buckets of money , but there is a lot of good cheap second hand stuff out there.
Power ? Is the boom high enough , about tit to shoulder high , and harness lines long enough , arms almost straight . Also make sure the lines are balanced left to right , should be about equal pull on both arms.
Downwind is harder, its a practice thing.
Without seeing what your doing I cant comment on your gybing.
A couple of tips mabee:
Start practice in very light wind,
Look around the corner to where you want to go and complete your turn ALL the way before flipping the sail,
When planing into a gybe , go down wind 30 degrees be aggressive and dive into the corner. It is scary at first but if you don't you kinda get stuck standing at the back of the board going directly downwind ,and its hard to finish the turn.
Windsurfing is not easy , But it is awesome , if you want easy try Kitesurfing.