Hello all, I am new here and have little to no experience with growing a bonsai, Ive taken some advice from the courses and I want to make a soil mixture. I see there are different sizes in the coarseness of akadama, pumis and lava rocks, some are Fine (1-3mm) Small(3-6mm) and Medium(5-8mm),
Is it of vital importance which grit size I choose? OR is it more dependent on my tree type? I have 3 nursery stock trees that i am starting with: Citrus (lemon), Thuja occidentalis (Cypress), and a Portulacaria afra.

I tend to use small size for small trees and the bigger size for bigger trees.

Bottom of the pot big size, then a layer medium and top soil is smallest size.

I agree with FrankC on the layering of sizes idea. Its important to remember that despite common logic a smaller pot will actually have a larger chance of water logging roots as the water table sits closer to the root ball. As long as what you do provides high drainage you should be fine either way. Following FrankC's advice will likely give you a great drainage set up.

I am sorry but layering will cause lots of problems and is not a great way.

The size of kernels determines how much water is retained. Finer keeps more water.
So in general smaller grain for smaller trees / pots, and smaller for trees that need more humid conditions. (My potentilla have very small kernels as they are the first to go dry, whereas my pines are in coarser substrate).