Generally, it's suggested that you start out with a 5/8 size razor. In terms of handling it's right between the bigger, heavier ones and the smaller, lighter ones. 5/8 being about average in handling for most beginners and leaving the room to concentrate on learning the continual basics of the craft. Lather building, learning the ins and outs of your face, learning to strop without ruining the edge. The "right" size will show it self to you once you get comfortable with getting decent shaves under your belt. And any chance you get to try another shaver's razors of different sizes it will help you zero in on your general decision. You
might eve decide that size doesn't matter, it's the quality of the shave that is more important. Time will tell.
In my case I started with a 4/8 and a 3/8. My general preference now is any size from 5/8 on up for the challenge of learning to master that particular size and grind. And grind isn't even on your mind as yet and that's another can of lather to think about.