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. Big or Small it is the holes in the harmonica that matter. All the holes are the same distance apart from each other on the harps.

One of the few difference that you will see is on the Hohner Special 20 the holes are a little larger. Pucker up and kiss your harp! A good way to practice making your mouth into the correct shape and size is to take a straw, cut it to just a couple of inches in length. Put it in your mouth and practice blowing and drawing. (Done in public or at work WILL get you strange looks!)

You can even use the straw to blow into and draw out of the harp and hear the sound. Practice and it will become second nature. It will not happen overnight, but done correctly you will notice an improvement pretty quickly.