I use very fine soil for my mame, since drainage really isn't an issue. The particles are between coarse sand and kitty litter. I don't repot them before a show, they live in their show pot all year round... but they sit in a sand tray because the sand evens out the moisture and the temperature of the pot.
For medium size trees, use a larger particle. From kitty litter size up to the size of a garden pea maybe. More or less.
For larger trees, you can use the larger particles, but you can use the same particles as the medium tree as well. Akadama is not large, nor is turface. The mix that I am currently using is 6 parts high quality akadama; 2 parts scoria; 2 parts pumice. None of the pieces are larger than the end of my little finger, and the akadama is fairly small. The scoria and the pumice are sifted using the small screen to remove the finest particles, and the largest particles are then sifted out and used for the Dreaded Drainage Layer.
This is being used for even the largest trees. Don't sift the akadama, except maybe with a very fine screen... or you won't have anything left.
This current recipe is being used because it is the suggestion of Kenji Miyata, as I am repotting trees that he will be styling. This is his method... drainage layer, sifting, and recipe... and the man whose trees I work on has Kenji come every few months for styling work.
The trees have really enjoyed the akadama, and there is nothing like it here. My own azaleas which I planted in a kanuma mix are just positively bursting with flowers, and have never looked this good. The few trees of my own that I have repotted with akadama in the mix are extremely happy. And I am using a shohin mix that includes small akadama and Japanese pumice, in which the shohin are thriving. I mix it with Aussie Gold fine seedling mix for the mame, to retain moisture in those teeeeny tiiiiiiiny pots.
So to answer your question.... use finer particles for the smallest pots, medium particles for good drainage in any pots larger than shohin, but some of it depends upon the soil components that you are using. In my very limited experience, I find that a soil mix which is too coarse in all of its components (like small orchid bark and the largest of the scoria sifting, and nothing else) does not encourage finer root growth. Instead, the roots are long and stringy, perhaps because the spaces between the particles is too large. That is merely an observation, not an established fact, but I found that roots which have grown in a mix that has some smaller components will be finer and denser.
The best thing to use is common sense and your own powers of observation. You will need to see how things grow in your area. We use akadama because it is so very dry here (less than 8" of rainfall this year, I think) but in a wetter climate that may be a mistake. Just experiment and observe. Really study the roots as you repot them, noting the growth and the health of the roots. Don't be afraid to poke around and tug on them and get in there with your nose. They should smell fresh. Eventually you will come upon the soil mix that you personally like, in the particle size that works best for you.
Joanie