The term "bigger board" here means that you at teh very minimum need some 50 liters above your weight in kg. Thus, for a 70 kg beginner a 120-130 liter board might work, nut anything smaller will prove to be quite challenging. It may make a lot of sense to start with a used board of more than 150 liters, and then sell it in order to switch donwn in size when you outgrow it. Do NOT assume that you can buy your dream board immediately, get the best possible gear for your skills, and then swap it for smaller or different material as you grow your skills.
Sailwise you are going to have to pick one or two sizes that matches the wind strength where you live, and where you are going to sail (is it sea or lake, where excatly?). Two sails with 5 and 6.5 m2 could be useful also when skills increase.
The thing with windsurfing is that it is much more difficult than it looks, but that is also the charn of it. There is always something to learn (even after 20+ years on board).