The ideal thing would be to go to a surf camp, ride their boards, and then decide what size/kind of board(s) you want to buy for yourself, because then you'll have firsthand experience of what you should be riding now, what you think you'll be riding in the near future, etc. The camp should have boards of various sizes and shapes for you to try out.
The reason everyone recommends larger boards is it's far preferable to buy a larger board than necessary (which really isn't going to hinder your progress much anyway) than to buy a board that's too small (which will definitely hinder your progress.) Bigger boards have more stability when you're up, but often the more important benefit is having a larger margin for error for catching waves. I've seen plenty of surfers riding boards that are too short for them, not because they can't stand on them, but because they are poor at catching and dropping into waves.
If you start on a bigger board and then quickly find you want to size down, the skills you've learned are going to carry over.