Selling and playing harmonicas for all these years, the 3 most common questions I get are: what harmonica(s) should I get, why and how do I learn to play by tomorrow? Let us just talk about the harmonica here...
Because the 10 hole diatonic is the standard of harmonicas and most widely used, start with it. What you are learning to do is blow or draw one note at a time, just takes practice. Size will not matter at all....