When you get your tree, take some pics for us. They are so helpful to everyone involved. It's hard to tell you exactly what size without seeing how the roots and foliage are currently.
If the roots are occupied all around the edges and looped around the bottom, it's time to be in a larger container. You should only be able to see a few roots on the sides and bottom.
Look at the bottom (or label on the container) to see what current size it is. Then we can tell you the next size up and how much CHC mix you'll need. A rough guess: An 18" container would be fine for a moderate canopy on a 4-5' tree. I prefer 20" and just set it a bit lower inside the container if it's rootball is too small. The next year I raise it and add fresh soil to the bottom and edges.
Are you buying the CHC from Crystal Company? If so, then I assume you've read the posts regarding soaking/CEx as well.
How to Remove the Tree to Examine Roots
Take out to tree completely from it's current pot (assuming it's not a huge 25"+ container) so you can see the roots and take another pic. Best to do this away from direct sun and don't let the exposed roots dry out (i.e. more than 20 minutes without misting). To remove it from the container, wait a couple days after your last watering. Then lean the container or place it on it's side, allow as much soil from the top and sides to gently fall out. If roots start coming downward, then stop there. Tap on the bottom and sides of the outside container to free the rootball up from the container itself. Then slowly pull off the container while holding the trunk. If the tree hasn't been freshly potted or is in heavy soil, it will likely just pop right out as a complete unit without any soil falling off and be in the shape of the container. If it won't budge, you'll need to tap/press harder on the bottom and sides to loosen it out. If it still won't move, then get a long knife and slide it along the edges of the container.
My apologies if you already know how to do all the above if it's written lower than your current skills and experience.
